Catch gas exit-via-signal

	gas/
	* as.h (gas_assert): Use abort.
	(as_assert): Remove.
	(signal_init): Declare.
	* as.c (main): Call signal_init.
	* messages.c: #include <signal.h>
	(as_assert): Delete.
	(as_abort): Allow NULL FILE.
	(signal_crash): New.
	(signal_init): Register fatal signal handlers.
	* configure.ac: Check for strsignal.
	* config.in: Rebuilt.
	* configure: Rebuilt.