Add the commit-id to the final tarball.

* Makefile.am (release): Pass --pax-options to tar.
* configure.ac: For the use of PAX tar.

This allows to retrieve the commit-id in a very effixient way using

bzcat tarball | git get-tar-commit-id

Drawback is that the PAX format is not available n older platforms.
it is a pitty that GNU tar does not allow to set the extended header
in non-pax mode.