...as is used by the Flatpak build.  Other settings only checked in that block
controlled by ENABLE_GTK, and thus potentially erronously left empty for
--disable-gtk --enable-gtk3 builds, are GTHREAD- and GTK_PRINT-related settings.
GTHREAD_LIBS is explicitly used in vcl/Library_vclplug_gtk3.mk even, but it
being empty apparently has no negative effects, so leave it at that for now.
On the GTK_PRINT-related settings, Caolan commented on IRC:  "ENABLE_GTK_PRINT
is probably best removed entirely I guess, it didn't work out and they're
redoing the dialog in upstream gtk again I'm told."

Change-Id: I7e026c0ac9b23e7ace2c2e92390bdcc3be7d39a3
Reviewed-on: https://gerrit.libreoffice.org/55127Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>