Index: kde-modules/KDECompilerSettings.cmake
===================================================================
--- kde-modules/KDECompilerSettings.cmake
+++ kde-modules/KDECompilerSettings.cmake
@@ -186,6 +186,49 @@
 # Language and toolchain features
 ############################################################
 
+include(CheckCXXCompilerFlag)
+include(CheckCCompilerFlag)
+
+# usage: kde_add_supported_cxx_compilerflag(-foo Compiler_Expression)
+# where Compiler_Expression is an expression you'd pass to if(), e.g.
+# CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 6.0.0
+# Adds the given flag to CMAKE_CXX_FLAGS if the current C++ compiler supports it
+# according to Compiler_Expression. When using clang on APPLE an actual test
+# is run after the Compiler_Expression check.
+macro (KDE_ADD_SUPPORTED_CXX_COMPILER_FLAG _FLAG)
+    # treat the arguments following _FLAG as a boolean expression
+    # to determine if we're dealing with a compiler/version that
+    # is expected to support _FLAG.
+    if (${ARGN})
+        if(APPLE AND CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+            # Clang on APPLE needs verification because of Apple's
+            # llvm versions which cannot be matched easily to
+            # stock llvm versions.
+            set(HASFLAG "${CMAKE_CXX_COMPILER_ID}++_ACCEPTS${_FLAG}")
+            check_cxx_compiler_flag(${_FLAG} ${HASFLAG})
+            if ({${HASFLAG}})
+                set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${_FLAG}")
+            endif()
+        else()
+            set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${_FLAG}")
+        endif()
+    endif()
+endmacro()
+
+macro (KDE_ADD_SUPPORTED_C_COMPILER_FLAG _RESULT _FLAG)
+    if (${ARGN})
+        if(APPLE AND CMAKE_C_COMPILER_ID MATCHES "Clang")
+            set(HASFLAG "${CMAKE_C_COMPILER_ID}_ACCEPTS${_FLAG}")
+            check_cx_compiler_flag(${_FLAG} ${HASFLAG})
+            if ({${HASFLAG}})
+                set(CMAKE_CX_FLAGS "${CMAKE_CX_FLAGS} ${_FLAG}")
+            endif()
+        else()
+            set(CMAKE_CX_FLAGS "${CMAKE_CX_FLAGS} ${_FLAG}")
+        endif()
+    endif()
+endmacro()
+
 # Pick sensible versions of the C and C++ standards.
 # Note that MSVC does not have equivalent flags; the features are either
 # supported or they are not.
@@ -369,20 +412,12 @@
     # Make some warnings errors
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=return-type")
 endif()
-if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR
+kde_add_supported_cxx_compiler_flag(-Wvla
+    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 "${CMAKE_CXX_FLAGS} -Wvla")
-endif()
-if ((CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.0) OR
+kde_add_supported_cxx_compiler_flag(-Wdate-time
+    (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.0) OR
     (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5))
-    include(CheckCXXCompilerFlag)
-    check_cxx_compiler_flag(-Wdate-time HAVE_DATE_TIME)
-    if (HAVE_DATE_TIME)
-        # -Wdate-time: warn if we use __DATE__ or __TIME__ (we want to be able to reproduce the exact same binary)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wdate-time")
-    endif()
-endif()
 
 # -w1 turns on warnings and errors
 # FIXME: someone needs to have a closer look at the Intel compiler options
@@ -486,12 +521,12 @@
     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")
+    kde_add_supported_cxx_compiler_flag(-fdiagnostics-color=always
+        ((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
@@ -71,8 +71,4 @@
    endif()
 endif()
 
-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()
+kde_add_supported_cxx_compiler_flag("-Wzero-as-null-pointer-constant" CMAKE_CXX_COMPILER_ID MATCHES "Clang")