Add a RLN command (relative ln), it can be used to create relative
symlinks using install's relative symlinks feature.

If you were going through hoops doing things like this to get relative
symlinks:

${LN} -sf ../lib/some/thing ${STAGEDIR}${PREFIX}/bin

You can now use full paths for both the source and destination:

${RLN} ${STAGEDIR}${PREFIX}/libexec/some/thing ${STAGEDIR}${PREFIX}/bin

One caveat is that the source must exist when this is run, because
install needs to run realpath(1) on it.

Sponsored by: Absolight