...and use PATH_VARS to make the config file work with absolute paths.
Two reasons to do this:
- DBUS_INTERFACES_INSTALL_DIR is marked deprecated
- Not hard-coding the packackage prefix is helpful on a multiarch
layout where the prefix is /usr/${host} but arch-independent files
should still be installed to /usr/share (i.e a level below the
prefix).
REVIEW: 125843
