Add the 'ENABLE_GTK3' and 'ENABLE_QT5' there
to allow using sth like

    #include <config_vclplug.h>
    #if ENABLE_GTK3
    // your code here
    #endif

for these as well.

Remove 'config_gtk3_kde5.h{,.in}', 'config_kde5.h{,.in}'
and 'config_qt5.h{,.in}'; they aren't included anywhere.
('QT5_HAVE_GLIB' is only used in Makefiles, so there's no
need to have it in a config header and 'KDE5_HAVE_GLIB' is
not used anywhere at all.)

Change-Id: Iea7906880f57dde782c50f2520e8d9b358f6bc82
Reviewed-on: https://gerrit.libreoffice.org/73204
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>