build: Move generated config.h into include/common/

config.h is a generated file with "build-features" defines.
We use it for several purposes:
o to check that compiler can do it's job
o to complement user-visible API between distributions
o to add compile-time options from .config global file

It's used in criu and soccr, but compel also needs such thing.

Previously, soccr has a link to config.h in criu includes,
but it would be much cleaner to move it to other headers,
that are shared between sub-projects into include/common.

Reported-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Tested-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>