Update Windows documentation following the plan in https://gist.github.com/notoraptor/ec67e03e7bb311b836e0e351a41b4505

Some other files have been also updated:
 * Add a |CompilerName| replacement rule to `requirements.inc` which is `gcc` for Windows/Linux and `Clang` for Mac.
 * Update inclusion rule of `requirements.inc` everywhere it is included to take account of the new replacement rule.
 * Update `install_generic.inc`. The libgpuarray stable version is now set to `v0.6.0`. I have also updated the download instructions because the actual instructions does not work on my Windows machine.

Some other updates and corrections to come (eespecially, how to configure libgpuarray on windows).