More config.h fixups for Meson
This should be the last of them from #1313 (closed). I don’t think any of this is worth a freeze break request for 2.58.0, so I’m targetting 2.60 with it. I might backport the GLIB_USING_SYSTEM_PRINTF
fix to 2.58.1 if it proves useful, although I doubt that many applications actually use that macro at all. Debian codesearch doesn’t throw up anything except GLib and packages which embed copies of it (cogl, pkg-config).
Closes #1313 (closed)