[build] Really use __builtin_offsetof() when available

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/stddef.h

 #define NULL ((void *)0)
 
 #undef offsetof
-#ifdef __compiler_offsetof
-#define offsetof(TYPE,MEMBER) __compiler_offsetof(TYPE,MEMBER)
+#if ( defined ( __GNUC__ ) && ( __GNUC__ > 3 ) )
+#define offsetof(TYPE, MEMBER) __builtin_offsetof(TYPE, MEMBER)
 #else
 #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
 #endif