Enable relative paths.

The documentation for the --with-external-tar was previously not
specific that it required an absolute path.  So rather than fix the
help text, convert the path to an absolute one.

Change-Id: I794cd77235e68a2270acd09895bf994527e847c4
Reviewed-on: https://gerrit.libreoffice.org/8146
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>