util: Add ATTRIBUTE_RETURNS_NONNULL.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
diff --git a/configure.ac b/configure.ac
index 023110e..9fbfe4d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -224,6 +224,7 @@
 AX_GCC_FUNC_ATTRIBUTE([malloc])
 AX_GCC_FUNC_ATTRIBUTE([packed])
 AX_GCC_FUNC_ATTRIBUTE([pure])
+AX_GCC_FUNC_ATTRIBUTE([returns_nonnull])
 AX_GCC_FUNC_ATTRIBUTE([unused])
 AX_GCC_FUNC_ATTRIBUTE([warn_unused_result])