New build targets for the global Makefile.

[valera-rozuvan]

@shervinemami @juliena82 Please be aware that I updated the global Makefile. Now you can use:

$ make windows

or

$ make linux

Also, I have included a clean target:

$ make clean

---

I have also updated the doc/build-instructions.md file.

[juliena82]

Good modification!