When using GCC, we always want GNU inline semantics. Fixes issues with

-Wsystem-headers and -std=c99, where GCC 4.2 would warn that it couldn't
give us C99 inline semantics.
ok guenther@, dcoppa@, millert@, deraadt@

src/include/ctype.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ctype.h,v 1.20 2007/03/17 21:38:14 espie Exp $	*/
+/*	$OpenBSD: ctype.h,v 1.21 2010/07/25 15:35:29 kettenis Exp $	*/
 /*	$NetBSD: ctype.h,v 1.14 1994/10/26 00:55:47 cgd Exp $	*/
 
 /*
@@ -59,7 +59,7 @@ extern const short	*_toupper_tab_;
 
 /* extern __inline is a GNU C extension */
 #ifdef __GNUC__
-#  if defined(__GNUC_STDC_INLINE__)
+#  if __GNUC_PREREQ__(4, 2)
 #define	__CTYPE_INLINE	extern __inline __attribute__((__gnu_inline__))
 #  else
 #define	__CTYPE_INLINE	extern __inline