diff --git a/lib/releaseme/requirement_checker.rb b/lib/releaseme/requirement_checker.rb
index 7e71b83..7cbe8d1 100644
--- a/lib/releaseme/requirement_checker.rb
+++ b/lib/releaseme/requirement_checker.rb
@@ -1,155 +1,163 @@
 #--
 # Copyright (C) 2015-2017 Harald Sitter <sitter@kde.org>
 #
 # This program is free software; you can redistribute it and/or
 # modify it under the terms of the GNU General Public License as
 # published by the Free Software Foundation; either version 2 of
 # the License or (at your option) version 3 or any later version
 # accepted by the membership of KDE e.V. (or its successor approved
 # by the membership of KDE e.V.), which shall act as a proxy
 # defined in Section 14 of version 3 of the license.
 #
 # This program is distributed in the hope that it will be useful,
 # but WITHOUT ANY WARRANTY; without even the implied warranty of
 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 # GNU General Public License for more details.
 #
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #++
 
 # NB: cannot use other files as everything else is meant to load this first.
 require_relative 'silencer'
 
 module ReleaseMe
   # Makes sure the runtime requirements of releasme are met.
   class RequirementChecker
     # Finds executables. MakeMakefile is the only core ruby entity providing
     # PATH based executable lookup, unfortunately it is really not meant to be
     # used outside extconf.rb use cases as it mangles the main name scope by
     # injecting itself into it (which breaks for example the ffi gem).
     # The Shell interface's command-processor also has lookup code but it's not
     # Windows compatible.
     class Executable
       attr_reader :bin
 
       def initialize(bin)
         @bin = bin
       end
 
       # Finds the executable in PATH by joining it with all parts of PATH and
       # checking if the resulting absolute path exists and is an executable.
       # This also honor's Windows' PATHEXT to determine the list of potential
       # file extensions. So find('gpg2') will find gpg2 on POSIX and gpg2.exe
       # on Windows.
       def find
         # PATHEXT on Windows defines the valid executable extensions.
         exts = ENV.fetch('PATHEXT', '').split(';')
         # On other systems we'll work with no extensions.
         exts << '' if exts.empty?
 
         ENV['PATH'].split(File::PATH_SEPARATOR).each do |path|
           path = unescape_path(path)
           exts.each do |ext|
             file = File.join(path, bin + ext)
             return file if executable?(file)
           end
         end
 
         nil
       end
 
       private
 
-      def executable?(path)
-        stat = File.stat(path)
-      rescue SystemCallError
-      else
-        return true if stat.file? && stat.executable?
-      end
+      class << self
+        def windows?
+          @windows ||= ENV['RELEASEME_FORCE_WINDOWS'] || mswin? || mingw?
+        end
 
-      def mswin?
-        @mswin ||= /mswin/ =~ RUBY_PLATFORM
-      end
+        private
+
+        def mswin?
+          @mswin ||= /mswin/ =~ RUBY_PLATFORM
+        end
 
-      def mingw?
-        @mingw ||= /mingw/ =~ RUBY_PLATFORM
+        def mingw?
+          @mingw ||= /mingw/ =~ RUBY_PLATFORM
+        end
       end
 
       def windows?
-        @windows ||= ENV['RELEASEME_FORCE_WINDOWS'] || mswin? || mingw?
+        self.class.windows?
+      end
+
+      def executable?(path)
+        stat = File.stat(path)
+      rescue SystemCallError
+      else
+        return true if stat.file? && stat.executable?
       end
 
       def unescape_path(path)
         # Strip qutation.
         # NB: POSIX does not define any quoting mechanism so you simply cannot
         # have colons in PATH on POSIX systems as a side effect we mustn't
         # strip quotes as they have no syntactic meaning and instead are
         # assumed to be part of the path
         # http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
         return path.sub(/\A"(.*)"\z/m, '\1') if windows?
         path
       end
     end
 
     # NOTE: The versions are restricted upwards because behavior changes in the
     # language can result in unexpected outcome when using releaseme. i.e.
     # you may end up with a broken or malformed tar. To prevent this, a change
     # here must be followed by running `rake test` to pass the entire test suite
     # Also see the section on bumping versions in the REAMDE.
     COMPATIBLE_RUBIES = %w[2.2.0 2.3.0 2.4.0 2.5.0].freeze
     REQUIRED_BINARIES = %w[svn git tar xz msgfmt gpg2].freeze
 
     def initialize
       @ruby_version = RUBY_VERSION
     end
 
     def check
       raise 'Not all requirements met.' unless check_ruby && check_binaries
     end
 
     private
 
     def check_ruby
       return true if ruby_compatible?
       print "- Ruby #{COMPATIBLE_RUBIES.join(' or ')} required."
       print "  Currently using: #{@ruby_version}"
       false
     end
 
     def check_binaries
       missing_binaries.each do |m|
         print "- Missing binary: #{m}."
       end.empty?
     end
 
     def print(*args)
       return if Silencer.shutup?
       puts(*args)
     end
 
     def ruby_compatible?
       COMPATIBLE_RUBIES.each do |v|
         return true if compatible?(v)
       end
       false
     end
 
     def missing_binaries
       missing_binaries = []
       REQUIRED_BINARIES.each do |r|
         missing_binaries << missing?(r)
       end
       missing_binaries.compact
     end
 
     def compatible?(a)
       Gem::Dependency.new('', "~> #{a}").match?('', @ruby_version)
     end
 
     def missing?(bin)
       return bin unless Executable.new(bin).find
       nil
     end
   end
 end