mesa: Detect and provide macros for function attributes pure and const.

These are really useful hints to the compiler in the absence of link-time
optimization, and I'm going to use them in VC4.

I've made the const attribute be ATTRIBUTE_CONST unlike other function
attributes, because we have other things in the tree #defining CONST for
their own unrelated purposes.

v2: Alphabetize.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)

src/util/macros.h

@@ -103,6 +103,17 @@ do {                       \
 #define assume(expr) assert(expr)
 #endif
 
+/* Attribute const is used for functions that have no effects other than their
+ * return value, and only rely on the argument values to compute the return
+ * value.  As a result, calls to it can be CSEed.  Note that using memory
+ * pointed to by the arguments is not allowed for const functions.
+ */
+#ifdef HAVE_FUNC_ATTRIBUTE_CONST
+#define ATTRIBUTE_CONST __attribute__((__const__))
+#else
+#define ATTRIBUTE_CONST
+#endif
+
 #ifdef HAVE_FUNC_ATTRIBUTE_FLATTEN
 #define FLATTEN __attribute__((__flatten__))
 #else
@@ -130,6 +141,15 @@ do {                       \
 #define PACKED
 #endif
 
+/* Attribute pure is used for functions that have no effects other than their
+ * return value.  As a result, calls to it can be dead code eliminated.
+ */
+#ifdef HAVE_FUNC_ATTRIBUTE_PURE
+#define PURE __attribute__((__pure__))
+#else
+#define PURE
+#endif
+
 #ifdef __cplusplus
 /**
  * Macro function that evaluates to true if T is a trivially