Doc/Makefile: error out if req tools absent

Make sure we have a clear message why build it failing, as there are
repeated questions at criu@ mailing list.

Before this patch:

	[kir@kirpad Documentation]$ make
	  GEN      criu.8
	make: *** [criu.8] Error 127

After:

	[kir@kirpad Documentation]$ make
	/bin/sh: line 1: xmltos: command not found
	make: *** [check] Error 1

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>