Define __attribute_const__

Signed-off-by: Leonid Lisovskiy <lly.dev@gmail.com>
Signed-off-by: Waldemar Brodkorb <wbx@uclibc-ng.org>

include/sys/cdefs.h

@@ -231,6 +231,12 @@
 # define __attribute_pure__ /* Ignore */
 #endif
 
+#if __GNUC_PREREQ (2,96)
+# define __attribute_const__ __attribute__((__const__))
+#else
+# define __attribute_const__     /* unimplemented */
+#endif
+
 /* At some point during the gcc 3.1 development the `used' attribute
    for functions was introduced.  We don't want to use it unconditionally
    (although this would be possible) since it generates warnings.  */