As well as checking for the existence of a GDC compiler, also validate
that it has also been built with libphobos, otherwise warn or fail with
the message that GDC is required to build d.
config/ChangeLog:
PR d/103528
* acx.m4 (ACX_PROG_GDC): Add check whether D compiler works.
ChangeLog:
* configure: Regenerate.