Add -l option to create symlinks for duplicates.

Most parts are stolen from abs2rel function:
https://github.com/kosaki/gtags/blob/master/libutil/abs2rel.c

This was a failing try to speedup building mixxx on PMBS:
(https://bugs.launchpad.net/mixxx/+bug/1303318)

But at least brp-25-symlink script doesn't have to relink anymore:
[  761s] calling /usr/lib/rpm/brp-suse.d/brp-25-symlink
[ 2080s] calling /usr/lib/rpm/brp-suse.d/brp-30-desktop