Alois Mahdal
da0869db93
Fix joining of DESTDIR and group root
Fix up the path joining logic:
* For obvious reasons, we panic if group's root is empty.
* DESTDIR can be empty, that's normal for installs such as when MKit
installs its own dogfood.
* Root (which sometimes starts with PREFIX) may or may not start with
slash; relative root such as that is also OK for installs like MKit
or TFKit.
* Furthermore, DESTDIR may or may not end with slash; we have to address
that and avoid joining the names.