Index: kde-modules/KDECompilerSettings.cmake
===================================================================
--- kde-modules/KDECompilerSettings.cmake
+++ kde-modules/KDECompilerSettings.cmake
@@ -186,6 +186,8 @@
 # Language and toolchain features
 ############################################################
 
+include(ECMAddCompilerFlag)
+
 # Pick sensible versions of the C and C++ standards.
 # Note that MSVC does not have equivalent flags; the features are either
 # supported or they are not.
@@ -369,20 +371,12 @@
     # Make some warnings errors
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=return-type")
 endif()
-if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR
-    (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5))
-    # -Wvla: use of variable-length arrays (an extension to C++)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wvla")
-endif()
-if ((CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.0) OR
-    (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5))
-    include(CheckCXXCompilerFlag)
-    check_cxx_compiler_flag(-Wdate-time HAVE_DATE_TIME)
-    if (HAVE_DATE_TIME)
-        # -Wdate-time: warn if we use __DATE__ or __TIME__ (we want to be able to reproduce the exact same binary)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wdate-time")
-    endif()
-endif()
+ecm_add_cxx_compiler_flags_conditionally(FLAGS -Wvla
+    CONDITION CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR
+        (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5))
+ecm_add_cxx_compiler_flags_conditionally(FLAGS -Wdate-time
+    CONDITION (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.0) OR
+        (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5))
 
 if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
    if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "5.0.0")
@@ -492,12 +486,12 @@
     set (CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} -Wl,--export-all-symbols")
 endif()
 
-if (CMAKE_GENERATOR STREQUAL "Ninja" AND
-    ((CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.9) OR
-     (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5)))
+if (CMAKE_GENERATOR STREQUAL "Ninja")
     # Force colored warnings in Ninja's output, if the compiler has -fdiagnostics-color support.
     # Rationale in https://github.com/ninja-build/ninja/issues/814
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fdiagnostics-color=always")
+    ecm_add_cxx_compiler_flags_conditionally(FLAGS -fdiagnostics-color=always
+        CONDITION ((CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.9) OR
+            (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5)))
 endif()
 
 include("${ECM_MODULE_DIR}/ECMEnableSanitizers.cmake")
Index: kde-modules/KDEFrameworkCompilerSettings.cmake
===================================================================
--- kde-modules/KDEFrameworkCompilerSettings.cmake
+++ kde-modules/KDEFrameworkCompilerSettings.cmake
@@ -61,18 +61,10 @@
   add_definitions(-DQT_STRICT_ITERATORS)
 endif()
 
-if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic")
-endif()
+ecm_add_cxx_compiler_flags_conditionally(FLAGS -pedantic
+    CONDITION CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
 
-if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
-   if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "5.0.0")
-      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wzero-as-null-pointer-constant" )
-   endif()
-endif()
+ecm_add_cxx_compiler_flags_conditionally(FLAGS -Wzero-as-null-pointer-constant
+    CONDITION CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "5.0.0")
 
-if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-   if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "5.0.0")
-      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wzero-as-null-pointer-constant" )
-   endif()
-endif()
+ecm_add_cxx_compiler_flags_conditionally(FLAGS -Wzero-as-null-pointer-constant CONDITION CMAKE_CXX_COMPILER_ID MATCHES "Clang")
Index: modules/ECMAddCompilerFlag.cmake
===================================================================
--- /dev/null
+++ modules/ECMAddCompilerFlag.cmake
@@ -0,0 +1,106 @@
+#=============================================================================
+# Copyright 2018      René J.V. Bertin <rjvbertin@gmail.com>
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+#
+# 1. Redistributions of source code must retain the copyright
+#    notice, this list of conditions and the following disclaimer.
+# 2. Redistributions in binary form must reproduce the copyright
+#    notice, this list of conditions and the following disclaimer in the
+#    documentation and/or other materials provided with the distribution.
+# 3. The name of the author may not be used to endorse or promote products
+#    derived from this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+# OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+# IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+# NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+# THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+include(CMakeParseArguments)
+include(CheckCXXCompilerFlag)
+include(CheckCCompilerFlag)
+
+# ECM_ADD_CXX_COMPILER_FLAGS_CONDITIONALLY(FLAGS <flag|flags>
+#     [CONDITION <conditional expression>]
+# )
+# add <flag> or <flags> to CMAKE_CXX_FLAGS if the compiler supports them.
+# Support is determined by the CONDITION expression if provided or by
+# querying the compiler directly otherwise. The compiler is also queried
+# when using a Clang C++ compiler on APPLE.
+# examples:
+# Check 
+# ecm_add_cxx_compiler_flags_conditionally(FLAGS -a -b -c CONDITION CMAKE_C_COMPILER_ID STREQUAL "GNU" OR CMAKE_C_COMPILER_ID MATCHES "Clang")
+# ecm_add_cxx_compiler_flags_conditionally(FLAGS -d -e -f)
+#
+# C-language equivalent:
+# ECM_ADD_C_COMPILER_FLAGS_CONDITIONALLY(FLAGS <flag|flags>
+#     [CONDITION <conditional expression>]
+# )
+
+
+function (ECM_ADD_CXX_COMPILER_FLAGS_CONDITIONALLY)
+    set(_OPTIONS_ARGS)
+    set(_ONE_VALUE_ARGS)
+    set(_MULTI_VALUE_ARGS FLAGS CONDITION)
+
+    cmake_parse_arguments(EASCXXFLAGS "${_OPTIONS_ARGS}" "${_ONE_VALUE_ARGS}" "${_MULTI_VALUE_ARGS}" ${ARGN} )
+    if(NOT EASCXXFLAGS_FLAGS)
+        message( FATAL_ERROR "ecm_add_cxx_compiler_flags_conditionally: 'FLAGS' is a required argument." )
+    endif()
+    # if the user provided supported_if conditions, evaluate them now:
+    if (NOT EASCXXFLAGS_CONDITION OR (${EASCXXFLAGS_CONDITION}))
+        # without conditions, or with Clang on APPLE we'll need to ask the compiler directly.
+        # (Clang on APPLE needs verification because of Apple's llvm versions which cannot be
+        # (matched easily to stock llvm versions.
+        if(NOT EASCXXFLAGS_CONDITION OR (APPLE AND CMAKE_CXX_COMPILER_ID MATCHES "Clang"))
+            # one flag at a time:
+            foreach(flag IN ITEMS ${EASCXXFLAGS_FLAGS})
+                # use a standardised and informative cached test variable
+                set(HASFLAG "${CMAKE_CXX_COMPILER_ID}++_ACCEPTS${flag}")
+                check_cxx_compiler_flag(${flag} ${HASFLAG})
+                if ({${HASFLAG}})
+                    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${flag}" PARENT_SCOPE)
+                endif()
+            endforeach()
+        else()
+            # all flags can be appended at once
+            string(REPLACE ";" " " FLAGS "${EASCXXFLAGS_FLAGS}")
+            set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${FLAGS}" PARENT_SCOPE)
+        endif()
+    endif()
+endfunction()
+
+function (ECM_ADD_C_COMPILER_FLAGS_CONDITIONALLY)
+    set(_OPTIONS_ARGS)
+    set(_ONE_VALUE_ARGS)
+    set(_MULTI_VALUE_ARGS FLAGS CONDITION)
+
+    cmake_parse_arguments(EASCFLAGS "${_OPTIONS_ARGS}" "${_ONE_VALUE_ARGS}" "${_MULTI_VALUE_ARGS}" ${ARGN} )
+    if(NOT EASCFLAGS_FLAGS)
+        message( FATAL_ERROR "ecm_add_c_compiler_flags_conditionally: 'FLAGS' is a required argument." )
+    endif()
+    if (NOT EASCFLAGS_CONDITION OR (${EASCFLAGS_CONDITION}))
+        if(NOT EASCFLAGS_CONDITION OR (APPLE AND CMAKE_C_COMPILER_ID MATCHES "Clang"))
+            # one flag at a time:
+            foreach(flag IN ITEMS ${EASCFLAGS_FLAGS})
+                set(HASFLAG "${CMAKE_C_COMPILER_ID}_ACCEPTS${flag}")
+                check_c_compiler_flag(${flag} ${HASFLAG})
+                if ({${HASFLAG}})
+                    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${flag}" PARENT_SCOPE)
+                endif()
+            endforeach()
+        else()
+            string(REPLACE ";" " " FLAGS "${EASCFLAGS_FLAGS}")
+            set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${FLAGS}" PARENT_SCOPE)
+        endif()
+    endif()
+endfunction()
+