Improved build procedures from CVS

I've committed changes in the GPS CVS tree that should remove all the
burden of having to use a partial stubs.c file depending on the version of
GNAT used to build GPS.

AFAIK, the build procedure should be simplified for any GNAT version.

Please let me know if you have trouble in your set up (and do not forget
to give details so that this can be investigated/reproduced), or if
you have success with the new (simplified) procedures, as listed in
the INSTALL file.

Arno