Calling Rscript from Makevars

I still haven't figured out how to do the Makevars thing,

It's very simple - just remove all targets from your Makefile and you have
Makevars since R will do the compilation and linking for you and correctly. If
you feel like it, you can override its behavior by adding one rule (e.g.,
all:) that lists your desired dependency and you can pass on to the R default
by depending on $(SHLIB).

I'm not clear on what to put in Makevars.
For example, in my Makefile.in, I have various header/include variables and
various library variables for compiling and linking different libraries that
could be in random locations on different platforms. In the Makefile.in, I
put these variables in the correct locations for compilation and linking. I
assume I can just change AC_CONFIG_FILES([src/Makefile]) to
AC_CONFIG_FILES([src/Makevars]) in autoconf.ac to get Makevars made from a
Makevars.in file. However, what I can't figure out is what my goal is to put
in a Makevars file. I've never seen any explanation of what Makevars files
do to accomplish all of the complexity of a Makefile. Does R use PKG_LIBS in
all compilation and linking steps or something? If so, then I guess I would
just have to set it equal to all of the other variables inside configure. I
haven't explicitly found that written somewhere, but I'm starting to wonder
if that's what it does. Anyway, if you could connect that dot for me, I'd
appreciate it. 

Thanks,
Sean

Cheers,
Simon