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.
hey timo, i don't think these `#if`s are correct. i've added comments which (i think) describe when and when not the blocks of code are included. one issues is that line 268 is only included when the OS is both windows and OS X at the same time. :P

    #ifdef __WIN32__
    #if defined(ARCH_32)
      // if windows and ARCH_32
    #define ARCH_SUBPATH "i386"
    #else 
      // if windows and not ARCH_32
    #define ARCH_SUBPATH "x64"
    #endif
    
    #if !defined(DONT_ADD_ENVIRONMENT_VARIABLES_FOR_R_PACKAGE)
    
         // if "windows" and "add environmental variables"
    
        env.insert("PATH", programDir.absoluteFilePath("R\\library\\RInside\\libs\\" ARCH_SUBPATH) + ";" + programDir.absoluteFilePath("R\\library\\Rcpp\\libs\\" ARCH_SUBPATH) + ";" + programDir.absoluteFilePath("R\\bin\\" ARCH_SUBPATH));
        env.insert("R_HOME", programDir.absoluteFilePath("R"));
        unsigned long processId = Process::currentPID();
        args << QString::number(processId);
    #undef ARCH_SUBPATH
    #elif __APPLE__
    
        // if "windows" and "dont add environmental variables" and "OS X"
    
        env.insert("R_HOME", programDir.absoluteFilePath("../Frameworks/R.framework/Versions/3.1/Resources"));
    #else
    
        // if "windows" and "dont add environmental variables" and not "OS X"
    
        env.insert("LD_LIBRARY_PATH", programDir.absoluteFilePath("R/lib") + ";" + programDir.absoluteFilePath("R/library/RInside/lib") + ";" + programDir.absoluteFilePath("R/library/Rcpp/lib"));
        env.insert("R_HOME", programDir.absoluteFilePath("R"));
    #endif
    #endif