make: Introduce compile time include/config.h generation

It's being reported that some systems (as Ubuntu 13.04) already
have struct tcp_repair_opt definition in their system headers.

| sk-tcp.c:25:8: error: redefinition of struct tcp_repair_opt
| sk-tcp.c:31:2: error: redeclaration of enumerator TCP_NO_QUEUE

So add a facility for compile time testing for reported entities
to be present on a system. For this we generate include/config.h
where all tested entries will lay and source code need to include
it only in places where really needed.

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