Rationale is the same as in described in commit
4aa6049062
("gbuild: treat $({QT,KF}5_CFLAGS) as cxxflags, not defines").
Change-Id: I1253e33118956b72561bd56bbc668ca345006c37
Reviewed-on: https://gerrit.libreoffice.org/73700
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
