D5017: Fix ecm_generate_pkgconfig_file compatibility with new cmake

[David Faure]

dfaure accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R240 Extra CMake Modules

BRANCH
  master

REVISION DETAIL
  https://phabricator.kde.org/D5017

To: xuetianweng, #frameworks, dfaure
Cc: #build_system