D23789: WIP: Add ECMGenerateExportHeaders, for improved handling of deprecated API

[Friedrich W. H. Kossebau]

kossebau added a dependent revision: D23800: RFC: Use ECMGenerateExportHeader 
to manage deprecated API better.

REPOSITORY
  R240 Extra CMake Modules

REVISION DETAIL
  https://phabricator.kde.org/D23789

To: kossebau
Cc: cgiboudeaux, kde-frameworks-devel, kde-buildsystem, LeGast00n, GB_2, 
bencreasy, michaelh, ngraham, bruns