make: Extend rules macros to pick sources from variable

This will allow us to generate rules where source
for object file is laying in some extenal directory
but compiled one is to be kept in some other directory.

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