Provide automatically generated version.h file

version.h includes defines to get version information about libdiscid:
DISCID_VERSION_MAJOR, DISCID_VERSION_MINOR and DISCID_VERSION_PATCH.

Signed-off-by: Sebastian Ramacher <sramacher@debian.org>
merge version defines to discid.h.in

This way it appears in the documentation and is easier to understand.
fix relative include in discid_private.h

That worked a long time, but with .in includes it breaks
out-of-source builds.
Merge branch 'discid_version'

See pull request #14