diff --git gcc/config/gnu.h gcc/config/gnu.h
index dddbcbf..4d9449e 100644
--- gcc/config/gnu.h
+++ gcc/config/gnu.h
@@ -34,7 +34,7 @@ along with GCC.  If not, see <http://www.gnu.org/licenses/>.
 	builtin_define ("__gnu_hurd__");	\
 	builtin_define ("__GNU__");		\
 	builtin_define_std ("unix");		\
-	builtin_define_std ("MACH");		\
+	builtin_define ("__MACH__");		\
 	builtin_assert ("system=gnu");		\
 	builtin_assert ("system=mach");		\
 	builtin_assert ("system=unix");		\