diff --git a/include/ac/string.h b/include/ac/string.h
index 9d07bf4b43..7a5931e37e 100644
--- a/include/ac/string.h
+++ b/include/ac/string.h
@@ -94,12 +94,12 @@ int (strncasecmp)();
 #define memcmp lutil_memcmp
 #endif
 
+/* GNU extension (glibc >= 2.1.91), only declared when defined(_GNU_SOURCE) */
 #ifndef HAVE_MEMRCHR
-	/* Actually, I think this is a GNU extension only */
-	void * lutil_memrchr(const void *b, int c, size_t len);
 #undef memrchr
 #define memrchr lutil_memrchr
 #endif /* ! HAVE_MEMRCHR */
+void * memrchr(const void *b, int c, size_t len);
 
 #define STRLENOF(s)	(sizeof(s)-1)