Fix compilation failure under ENABLE_CHECKING.

 src/alloc.c [ENABLE_CHECKING]: Include signal.h, since we need SIGABRT.

Fixes: debbugs:12327

src/ChangeLog

+2012-09-07  Eli Zaretskii  <eliz@gnu.org>
+
+	* alloc.c [ENABLE_CHECKING]: Include signal.h, since we need SIGABRT.
+
 2012-09-07  Dmitry Antipov  <dmantipov@yandex.ru>
 
 	* alloc.c (mark_buffer): Revert unsafe marking optimization.

src/alloc.c

@@ -26,6 +26,10 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include <limits.h>		/* For CHAR_BIT.  */
 #include <setjmp.h>
 
+#ifdef ENABLE_CHECKING
+#include <signal.h>		/* For SIGABRT. */
+#endif
+
 #ifdef HAVE_PTHREAD
 #include <pthread.h>
 #endif