devel/gps: Unbreak build after GDB update

The GDB version was hardcoded which was a bad idea. Update the makefile
to dynamically determine the current version of GDB for the PORTGDB
definition. While here, update RUN_DEPENDS to include gprbuild which is
a fix that I've held on to for a while. Also, gnat_util is no longer
needed to build GPS so remove that dependency as well.

Reported by: pkg-fallout