util: add MAYBE_UNUSED for config dependent variables

This is mostly for variables that are only used in asserts and cause
unused-but-set-variable warnings in release builds. Could just use
UNUSED directly, but MAYBE_UNUSED should be less confusing and is
similar to what the Linux kernel has.

And yes __attribute__((unused)) can be used on variables on both GCC 4.2
(oldest supported by mesa) and clang 3.0 (just some random old version,
not sure what's the minimum for mesa).

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Chad Versace <chad.versace@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/util/macros.h

@@ -204,6 +204,8 @@ do {                       \
 #define UNUSED
 #endif
 
+#define MAYBE_UNUSED UNUSED
+
 #ifdef HAVE_FUNC_ATTRIBUTE_WARN_UNUSED_RESULT
 #define MUST_CHECK __attribute__((warn_unused_result))
 #else