Compatible make.m for R2014a

- get only the selected C++ compiler (issue #112)
- add detection of VS2013 compiler
- simplified construction of `opencv_path`. This gives the user more flexibility for compiling mexopencv against a manually built OpenCV from sources (that way user can specify a custom directory tree as described [here](https://github.com/kyamagu/mexopencv#2-install-opencv-library)).