Modify the platform-specific makefiles so that macro 'rpath' is defined

Modify the platform-specific makefiles so that macro 'rpath' is defined
in terms of macro 'rpathdir', as I proposed a few weeks ago.  In itself
this commit shouldn't change the behavior at all, but it opens the door
to using special rpaths for the PL shared libraries, as seems to be
needed for plperl in particular.