Log message:
(1) Publicly export the value of _OPSYS_RPATH_NAME as RPATH_FLAG;
    Makefiles simply need to use this value often, for better or for
    worse.

(2) Create a new variable FIX_RPATH that lists variables that should
    be cleansed of -R or -rpath values if ${_USE_RPATH} is "no".  By
    default, FIX_RPATH contains LIBS, X11_LDFLAGS, and LDFLAGS, and
    additional variables may be appended from package Makefiles.