libgcobol: Heed --enable-libgcobol

libgcobol: Heed --enable-libgcobol

If some target isn't listed as supported in configure.tgt,
--enable-libgcobol cannot override that.  However, that's what should
happen just like an explicit --enable-languages=cobol forces the
frontend to be built.

This patch, shamelessly adapted from libphobos, does just that.

Tested on amd64-pc-solaris2.11, sparcv9-sun-solaris2.11, and
x86_64-pc-linux-gnu.

2025-04-08  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	libgcobol:
	* configure.ac: Handle --enable-libgcobol.
	Let it override LIBGCOBOL_SUPPORTED.
	* configure: Regenerate.