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

[René J . V . Bertin]

rjvbb set the repository for this revision to R240 Extra CMake Modules.

REPOSITORY
  R240 Extra CMake Modules

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

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