Make Rscript work for linux binary package, and use it to obtain the build variables

I *think* this will actually also make it possible to build against any other installed version of R, but I didn't test it so there's probably a bit of work to do for that. I just wanted to send an as-small-as-possible pull request with only "a view towards" linking against system R. Hopefully, you can review and merge this easily.
@tkluck, actually, i think i've changed my mind. i don't think i can make JASPEngine robustly self-contained. it will always require the setting of PATH (under windows at least), so it can find R.dll, and RInside.dll. I was going to add this setting of PATH to the initialization of JASPEngine, but of course it tries to load these dlls before the executable begins (catch 22).
so i think i'll stick with enginesync.cpp setting these variables before invoking JASPEngine.