config: Add check whether D compiler works (PR103528)

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.