config.h: enabled RET_NONNULL for GCC

Author: firewave

lib/config.h

@@ -121,11 +121,11 @@
 #  define DEPRECATED
 #endif
 
-// TODO: GCC apparently also supports this but there is no documentation on it
 // returns_nonnull
 #if __has_cpp_attribute (gnu::returns_nonnull)
 #  define RET_NONNULL [[gnu::returns_nonnull]]
-#elif (defined(__clang__) && ((__clang_major__ > 3) || ((__clang_major__ == 3) && (__clang_minor__ >= 7))))
+#elif (defined(__clang__) && ((__clang_major__ > 3) || ((__clang_major__ == 3) && (__clang_minor__ >= 7)))) \
+    || (defined(__GNUC__) && (__GNUC__ >= 9))
 #  define RET_NONNULL __attribute__((returns_nonnull))
 #else
 #  define RET_NONNULL