We really only need the logind API which can be provided either by
libsystemd or libelogind.
This also pulls out finding a logind provider from the native backend
only code because we make use of it in the generic backend code as well.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4223>
Allowing to disable font rendering integration, making it possible to
build Mutter without pango/harfbuzz/fribidi dependencies.
This commit also adds a new clutter-pango header that is used to include
pango specific bits.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4106>