mesa: Replace a priori knowledge of gcc attributes with configure tests.

Note that I had to add support for testing the packed attribute to
m4/ax_gcc_func_attribute.m4.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com> [C bits]
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/util/macros.h

@@ -75,24 +75,27 @@ do {                        \
 #define unreachable(str)
 #endif
 
+#ifdef HAVE_FUNC_ATTRIBUTE_FLATTEN
+#define FLATTEN __attribute__((__flatten__))
+#else
+#define FLATTEN
+#endif
 
-#if (__GNUC__ >= 3)
+#ifdef HAVE_FUNC_ATTRIBUTE_FORMAT
 #define PRINTFLIKE(f, a) __attribute__ ((format(__printf__, f, a)))
 #else
 #define PRINTFLIKE(f, a)
 #endif
 
-
 /* Used to optionally mark structures with misaligned elements or size as
  * packed, to trade off performance for space.
  */
-#if (__GNUC__ >= 3)
+#ifdef HAVE_FUNC_ATTRIBUTE_PACKED
 #define PACKED __attribute__((__packed__))
 #else
 #define PACKED
 #endif
 
-
 #ifdef __cplusplus
 /**
  * Macro function that evaluates to true if T is a trivially