build: fix handling of GTK requirement if none was given
build: fix handling of GTK requirement if none was given

Previously the logic defaulted to requiring GTK3, but that broke
when doing 'make distcheck' if you didn't have GTK3.  So if the
user didn't give a specific GTK version, just pick one
automatically.

lkundrak/network-manager-openconnect,lkundrak/network-manager-openconnect