Directly use 'ENABLE_GTK3' instead, s.a. discussion at
https://gerrit.libreoffice.org/#/c/72968/ .

Change-Id: I9713cd942e3f7308974eebc432dce0d2bc5adadc
Reviewed-on: https://gerrit.libreoffice.org/73270
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>