Better detection of the cleanup attribute.

Use g_macro__has_attribute to detect it instead of
hardcoding __GNUC__ || __clang__. This adds support
for a few compiler and is consistent with the rest
of the gmacros.h file.

docs/reference/glib/glib-sections.txt

@@ -783,6 +783,7 @@ g_macro__has_attribute_fallthrough
 g_macro__has_attribute_may_alias
 g_macro__has_attribute___noinline__
 g_macro__has_attribute_warn_unused_result
+g_macro__has_attribute_cleanup
 </SECTION>
 
 <SECTION>