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.
At least on Ubuntu/Debian boost is installed into `/usr/include`. So there's a directory `/usr/include/boost` and because `/usr/include` is in the default search path and you include things like `<boost/foreach.hpp>`, there's no need to add extra library search paths.

Not sure where boost installs if you just do `./configure && make && sudo make install` but I would be surprised if it is anything different from `/usr/local/include`. Where did you find the `/opt/local/include` ?