Use XDG_CURRENT_DESKTOP to check if GNOME is running.
Previously the existence of a `gtk_shell*` interface was used however
the WAYFIRE compositor provides this interface which made GHOST require
LIBDECOR.
Initializing LIBDECOR was hanging as the initial configuration was
never seen. From a quick check it doesn't seem like a bug in Blender's
LIBDECOR use, the issue may be in either LIBDECOR or WAYFIRE.
Whatever the case, LIBDECOR isn't needed so workaround the bug by
changing how GNOME is detected.
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