Do not segfault just because an error has not been given

Signed-off-by: Mattias Andrée <maandree@operamail.com>
Try to use DISPLAY=:0 if not in X

Signed-off-by: Mattias Andrée <maandree@operamail.com>
Fix #67: Geoclue should pull in Glib as dependency

Add checks in configure.ac for Glib and pull in through Makefile.am when
Geoclue support is enabled.
Fix #66: Do not distribute redshift-gtk, only redshift-gtk.in
Fix #64: Fix Geoclue support when run outside of X

Fix check of error value returned.
Geoclue requires DISPLAY to be set, so try DISPLAY=:0 if not already set.