ada: Require successful build of xsnamest

ada: Require successful build of xsnamest

While experimenting, I introduced a compilation error into xsnamest.
This took a little while to track down because, while the error was in
the log, the build did not stop.  This patch changes Make-generated.in
to require a successful build of this program.

gcc/ada/

	* Make-generated.in (ada/stamp-snames): Check result of
	gnatmake.