util: Add ATTRIBUTE_RETURNS_NONNULL.

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

m4/ax_gcc_func_attribute.m4

@@ -53,6 +53,7 @@
 #    optimize
 #    packed
 #    pure
+#    returns_nonnull
 #    unused
 #    used
 #    visibility
@@ -76,6 +77,9 @@
 
 #serial 2
 
+# mattst88:
+#     Added support for returns_nonnull attribute
+
 AC_DEFUN([AX_GCC_FUNC_ATTRIBUTE], [
     AS_VAR_PUSHDEF([ac_var], [ax_cv_have_func_attribute_$1])
 
@@ -175,6 +179,9 @@ AC_DEFUN([AX_GCC_FUNC_ATTRIBUTE], [
                 [pure], [
                     int foo( void ) __attribute__(($1));
                 ],
+                [returns_nonnull], [
+                    int *foo( void ) __attribute__(($1));
+                ],
                 [unused], [
                     int foo( void ) __attribute__(($1));
                 ],