Without direction, GPRconfig will create a cgpr file at the standard
$PREFIX/lib/gnat repository. This is the cause of the builder
failures because it touches a part of the userland it's not supposed to.
GPRconfig should be modified not to do this; it is never desirable.
In the meantime, the fix for APQ is to specify .cgpr targets which do
not exist so GPRconfig creates them there instead of $PREFIX/lib/gnat.
Reported by: swills
bfd732afd7