build: Use subdir-objects with Autotools

We're eventually going to drop Autotools, but in the meantime we should
probably use idiomatic options and reduce warnings.

GLib is pretty much already safe for subdir-objects to be enabled,
except in the GIO tests, where the build references files that are
generated in a different level. For that, we can use the same solution
employed by GTK+, and link the appropriate file in the right
sub-directory.

https://bugzilla.gnome.org/show_bug.cgi?id=788989