Build on Windows using MSVC, nmake

[chyndman]

These commits allow for building on Windows. I based this on my ExDoc work and submitted a PR here per @josevalim's request. The same steps to build hold:

1. Have Erlang and Elixir on system and in Path variable.
2. Git clone devinus/markdown into your favorite folder.
3. Install Visual Studio 2013 (Express should suffice, I'm using Ultimate).
4. In a Command Prompt run "C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\vcvarsall.bat" _ARCH_, where _ARCH_ is either x86 (if you installed 32-bit Erlang) or amd64 (64-bit Erlang).
5. cd into the repo folder and run mix compile.

Requires elixir-lang/elixir #2426.

[josevalim]

Thank you! Quick question: what does the "C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\vcvarsall.bat" _ARCH_ command does?

[devinus]

Probably injects ENV vars into your session.

[devinus]

MSVC comes with a MSVC version of the console as a shortcut which I assume does this by default when run, but not everybody wants to run cmd.exe when on Windows.

[chyndman]

@devinus Correct, it sets up environment variables. Calling it manually makes sure the architecture matches Erlang (the shortcut uses x86). Unfortunately variables only stay for the life of the cmd.exe shell which called the batch file.

[josevalim]

Thanks @devinus and @chyndman. Do you folks think we should add a note to the README telling developers on windows they need to have VS Express installed (with a link pointing to the install instructions) and a note with the command to run?