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.