Use g_get_current_dir()

get_current_dir_name() is a non-portable GNU extension.  Use
g_get_current_dir() instead, now that it does the same thing (ie:
respecting the value of PWD).

Bump GLib dependency to make sure we get the new version of the
function.

https://bugzilla.gnome.org/show_bug.cgi?id=705902