These days pcc defines __GNUC__ and we don't support gcc2. Also needed

for upcoming CompCert port.
Final version of the diff is from kettenis@ with input from jsg@ and tedu@.
ok kettenis@, jsg@, "I agree" millert@

src/include/stdbool.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: stdbool.h,v 1.6 2014/03/16 18:38:30 guenther Exp $ */
+/* $OpenBSD: stdbool.h,v 1.7 2015/09/04 23:47:09 daniel Exp $ */
 
 /*
  * Written by Marc Espie, September 25, 1999
@@ -10,8 +10,9 @@
 
 #ifndef __cplusplus
 
-#if (defined(__GNUC__) && __GNUC__ >= 3) || defined(__PCC__)
-/* Support for _C99: type _Bool is already built-in. */
+#if defined(__GNUC__) || \
+	(defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901)
+/* Support for C99: type _Bool is already built-in. */
 #define false	0
 #define true	1