Better checks for static_assert availability

When not in c++ or c11 mode:
- check for _Static_assert support in clang with __has_extension
- gcc implements _Static_assert starting with 4.6.0
- as a fallback, use a forward decl instead of ((void)0) so that
  static_assert can be used at file scope (scope issue pointed out by
  guenther@)

ok jsg@

lib/mesa/include/c11_compat.h

@@ -12,16 +12,22 @@
    /* Already C11 */
 #else
 
+#ifndef __has_extension
+#define __has_extension(x) 0
+#endif
 
 /*
  * C11 static_assert() macro
  * assert.h only defines that name for C11 and above
  */
 #ifndef static_assert
-#ifdef __clang__
+#if defined(__clang__) && __has_extension(c_static_assert)
+#define static_assert _Static_assert
+#elif defined(__GNUC__) && ((__GNUC__ == 4 && __GNUC_MINOR__ >= 6) || (__GNUC__ > 4))
+#warning 
 #define static_assert _Static_assert
 #else
-#define static_assert(...) ((void)0)
+#define static_assert(cond, mesg) struct _C11_COMPAT_H_static_assert_unused
 #endif
 #endif