mesa: Add a MUST_CHECK macro for __attribute__((warn_unused_result)).

In the kernel, this is called __must_check; all our attribute macros in
Mesa appear to be uppercase, so I went with that.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/util/macros.h

@@ -182,6 +182,12 @@ do {                       \
 #define UNUSED
 #endif
 
+#ifdef HAVE_FUNC_ATTRIBUTE_WARN_UNUSED_RESULT
+#define MUST_CHECK __attribute__((warn_unused_result))
+#else
+#define MUST_CHECK
+#endif
+
 /** Compute ceiling of integer quotient of A divided by B. */
 #define DIV_ROUND_UP( A, B )  ( (A) % (B) == 0 ? (A)/(B) : (A)/(B)+1 )