There were a couple of custom paths which could end up being relative,
rather than absolute, due to not properly prefixing them with
`get_option('prefix')`.

The use of `join_paths()` here correctly drops all path components
before the final absolute path in the list of arguments. So if someone
configures GLib with an absolute path for `gio_module_dir`, that will be
used unprefixed; but if someone configures with a relative path, it will
be prefixed by `get_option('prefix)`.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1919