D16894: [ECM] use a macro to add compiler flags conditionally

[René J . V . Bertin]

rjvbb edited the test plan for this revision.

REPOSITORY
  R240 Extra CMake Modules

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

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