D16894: [ECM] use a macro to test compiler flag support

[René J.V. Bertin]

rjvbb created this revision.
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REVISION SUMMARY
  It is crucial not to add unsupported compiler options because they can cause CMake's `check_<lang>_compiler_flags` macro to fail, causing problems like https://bugs.kde.org/show_bug.cgi?id=401018.
  To prevent this, run an actual test when using clang on Mac.
  
  This is almost unavoidable because determining compiler flag availability from compiler ID + version can be tricky to near impossible on Mac. Apple's llvm version numbers that are ahead of stock llvm versions and not easy to match. In addition, not all cmake versions distinguish between stock clang and "AppleClang" and those that do need policy `CMP0025` set to `NEW`. That policy cannot be set after the `project` statement in the toplevel cmake file (= not in an ECM module).
  
  This patch introduces a set of macros to simplify writing reliable conditional code for adding compiler flags. If will do the usual compiler ID + version checking to determine if a flag should be supported; when using Clang on Mac it will in addition use `check_<lang>_compiler_flag()`. This should be safe because Apple's llvm versions have always been ahead of stock versions; false negatives should thus never occur.
  The C++ version of the new macro is used where possible.
  
  BUG: 401018

TEST PLAN
  See https://bugs.kde.org/show_bug.cgi?id=401018 ; without this patch the project fails to build using Xcode compiler versions because the generated kcddb_export.h header does not match the visibility settings.

REPOSITORY
  R240 Extra CMake Modules

REVISION DETAIL
  https://phabricator.kde.org/D16894

AFFECTED FILES
  kde-modules/KDECompilerSettings.cmake
  kde-modules/KDEFrameworkCompilerSettings.cmake

To: rjvbb, #build_system
Cc: kde-frameworks-devel, kde-buildsystem, #build_system, michaelh, ngraham, bruns
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