Update configure to check for a recent gnat Ada compiler.

Update configure to check for a recent gnat Ada compiler.

GCC fails to bootstrap when configured with --enable-languages=all on
machines that have older versions of GNAT installed as the system Ada
compiler.  In configure, it's not sufficient to check whether gnat is
available, but whether a sufficiently recent version of GNAT is
installed.  This patch tweaks config/acx.m4 so that conftest.adb also
contains a reference to System.CRTL.int64 as required by the current
version of gcc/ada/osint.adb.  This fixes the build when the system
Ada is GNAT v4.8.5 (on Redhat 7) by disabling ada, but continues to
work fine when the system Ada is GNAT v11.3.1.

2022-08-01  Roger Sayle  <roger@nextmovesoftware.com>
	    Arnaud Charlet  <charlet@adacore.com>

config/ChangeLog
	* acx.m4 (AC_PROG_GNAT): Update conftest.adb to include
	features required of the host gnat compiler.

ChangeLog
	* configure: Regenerate.