glib.setupHook: fix gsettings-schemas location

GLib setup hook expects GSettings schemas to be installed in ${!outputLib}
and tries to move them to gsettings-schemas/$name subdirectory to prevent
conflicts. But the schemas will only end up in the library output when
the build system recognizes makeFlags set by the setup hook, and in that case
the move is not necessary, since the flag already includes the subdirectory.

Normally, this is not an issue, since most packages relying on GSettings
schemas either still use Autotools with gsettings.m4, or do not have a lib
output set. But with the promulgation of multiple outputs in Nixpkgs and
more and more projects switching to Meson, the issue will become increasingly
common.

We first noticed this problem with nm-applet.

Closes https://github.com/NixOS/nixpkgs/issues/45043