Add githead helper

shl_githead is a short helper to put the build-git-revision into compiled
objects as string. It is bundled to a Makefile.am snippet that takes care
of it.

Note that it is highly optimized to only rebuild if the revision really
changed. Furthermore, it is created to work with dist-tarballs and VPATH
builds.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Include stddef.h explicitly for offsetof()

It seems that stdlib.h does not automatically include stddef.h. Via some
other paths, it did get included so far, but outside projects may fail.
Hence, include it explicitly.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>