Use pkg-config for building against R on linux

Hi Jonathon!

I tried building JASP on Ubuntu against the default installation of R that's in Ubuntu (and Debian). I needed to make a few adjustments to the qmake files.

The modifications use the "official" way to get the install path for packages (pkg-config for linking against R; reading R's internal variables for getting the location of Rinside's library path). I'm using `-rpath` to make sure that the shared libraries for Rinside are found at runtime.

This is a pull request, but I wouldn't merge just yet; I get the following error when running JASPEngine:

    tkluck@ultrabook-tjk:~/src/jasp-desktop$ LANG=C ./JASPEngine 
    terminate called after throwing an instance of 'boost::interprocess::interprocess_exception'
      what():  No such file or directory

I haven't had time to investigate yet.