gnattools build machinery uses just-build xgcc and xg++ as $(CC) and
$(CXX) in native builds.  However, if C and C++ languages are not
enabled, it won't find them.  So, enable C and C++ if Ada is enabled.
Most of the time, this is probably no big deal: C is always enabled
anyway, and C++ is already enabled for bootstraps.

We need not enable those for cross builds, however.  At first I just
took the logic from gnattools/configure, but found it to be lacking:
it would use the just-built tools even in cross-back settings, whose
tools just built for the host would not run on the build machine.  So
I've narrowed down the test to rely on autoconf-detected cross-ness
(build->host only), but also to ensure that host matches build, and
that target matches host.

I've considered sourcing ada/config-lang.in from within
gnattools/configure, and testing lang_requires as set by it, so as to
avoid a duplication of tests that ought to remain in sync, but decided
it would be too fragile, as ada/config-lang.in does not expect srcdir
to refer to gnattools.

for  gcc/ada/ChangeLog

	PR ada/81878
	* gcc-interface/config-lang.in (lang_requires): Set to "c c++"
	when gnattools wants it.

for  gnattools/ChangeLog

	PR ada/81878
	* configure.ac (default_gnattools_target): Do not mistake
	just-built host tools as native in cross-back toolchains.
	* configure: Rebuilt.

From-SVN: r266290