Our whole system of Makefiles are integrated from top to bottom,
meaning:

 1. The paths in sub-makefiles are relative to the top source dir.
 2. Sub-makefiles are executed via make $(build)=<SUBDIR> <TARGET>

For some reason, makefiles under lib/ are the exclusion. Let's fix it.

Side effect: you can now build any individual target under lib/,
for example, "make lib/c/libcriu.so" works.

[v2: use the .FORCE, thanks to dsafonov@]
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>