make: Allow to install in custom dirs

There is no way to redefine install paths into
some custom ones (say plain /usr). Fix it by
using conditional inits.

Reported-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>