patch mostly by Peter Kümmel, bug #8211

	* dbus/dbus-sysdeps-unix.c: new file, which splits out
	unix-specific stuff in dbus-sysdeps.c

	* dbus/dbus-sysdeps.c: don't include any UNIX-only headers,
	and move unix-specific stuff to dbus-sysdeps-unix.c

	* configure.in: check HAVE_ERRNO_H