ability to compile libsoundio source with MSVC

@hotgloupi [says](https://github.com/andrewrk/libsoundio/pull/48#issuecomment-159247603)

> > What is the use case for compiling libsoundio with visual c++?
>
> Well, there can be many use cases for the users:
> - Seamlessly build libsoundio as part of the project
> - Honor the build variants (threading mode, debug mode, static vs dynamic
> runtime, debug runtime mode)
> - Being able to compile the HEAD in a windows environment (without waiting
> for an external cross-compiled binary)
> - Using the VS debugger
> - Being able to hack libsoundio in VS (like to make Windows store
> friendly, or to build for IOS/Android from VS, etc.)
>
> There might be more advantages, I'm not a windows guys, but I know that
> many multimedia guys are working / targeting a windows environment.
>
> Stating that the project only compiles using gcc will stop many windows
> users to use/contribute to libsoundio.
>
> That being said, I will make it compile on visual c++, I was only asking
> for some guidance, even if my work won't ever be merged back.