Use g_credentials_get_unix_pid() instead of home baked function

This function is available in GIO since 2.36

Thanks to Petr Salinger <Petr.Salinger@seznam.cz> for the patch
Debian Bug: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=692739

https://bugzilla.gnome.org/show_bug.cgi?id=687921
Add configure option to specify gnome-settings-daemon location

Add --with-gnome-settings-daemon-directory option to allow one to pass
location for the gnome-settings-daemon executable used by the chooser

https://bugzilla.gnome.org/show_bug.cgi?id=645918
Don't use both AM_GNU_GETTEXT and IT_PROG_INTLTOOL

When using IT_PROG_INTLTOOL using the gettect automake macros isn't
necessary. To make matters worse, when mixing the marcos both try to
generate po/Makefile.in.in. Recent versions of intltool detect when
gettext won and bails when building.

https://bugzilla.gnome.org/show_bug.cgi?id=711818
Add an option to use GDM as a standalone XDMCP server

This patch add an option (xdmcp/ShowLocalGreeter) to disable the local X
console and turn GDM into a simple XDMCP server

Thanks to Patrick Monnerat and Josselin Mouette for the patch

https://bugzilla.gnome.org/show_bug.cgi?id=567522