* gnu/packages/gnome.scm (gdm)[arguments]: Update pre-configure phase to
make GDM get the configuration file path from an environment variable.
* gnu/services/xorg.scm (gdm-etc-service): Remove function.
(gdm-configuration-file): New function.
(gdm-shepherd-service): Set GDM_CUSTOM_CONF before invoking GDM.
(gdm-service-type)[extensions]: Remove etc-service-type extension.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>