use relative paths for installing artifacts to allow packaging
use relative paths for installing artifacts to allow packaging

CPack is sensitive to absolute paths since it is using `make DESTDIR=...`
to install in a local directory, prefixed appropriately. However, when
the destinations are absolute paths, cpack uses those paths directly
and fails packaging generic archives.

hanw/p4c,p4lang/p4c,hanw/p4c,mbudiu-vmw/p4c-clone,hanw/p4c,p4lang/p4c,mbudiu-vmw/p4c-clone,p4lang/p4c,hanw/p4c,p4lang/p4c,mbudiu-vmw/p4c-clone,p4lang/p4c,mbudiu-vmw/p4c-clone