use __builtin_offsetof for gcc4 and superior, it avoids a gcc4 broken build with template use of offsetof()

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@20669 a95241bf-73f2-0310-859d-f6bbb57e9c96

headers/posix/stddef.h

@@ -5,7 +5,11 @@
 */
 #include <null.h>
 
+#if defined(__GNUC__) && __GNUC__ > 3
+#define offsetof(type,member)   __builtin_offsetof(type, member)
+#else
 #define offsetof(type,member) ((size_t)&((type*)0)->member)
+#endif
 
 typedef long ptrdiff_t;