Since vendor Makefiles make use of special GNU make features, we can safely
rely on GNU make being in operation. GNU make provides the variable
MAKEFILE_LIST from which the relative path can be extracted at which
Makefile_var was found. This exactly is the path of the contrib directory,
such that the user (calling Makefile) needs not set this variable.
Currently the calling Makefiles still set a variable CONTRIBDIR which then
simply is overwritten in Makefile_var. Such the setting of CONTRIBDIR is