configure: let configure script generate config.h file

While we're at it, also let configure script generate the config.h
file. The generation of it is currently quite static, but that could
change in future. While at it, we also generate a GITVERSION define,
that we will use for ``--version'' output of the individual tools for
being able to provide better support since we can see what concrete
commit people are using when building from Git repo.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>