Turn on libmount by default on linux

The libmount code produces somewhat different results than the older
Linux code that would parse /proc/mounts; for example, bind mounts
appear in the libmount output. To try and get as many GLib users
as possible to have the same behavior, on Linux, make GLib error out
on missing libmount unless --disable-libmount is passed.

https://bugzilla.gnome.org/show_bug.cgi?id=771438