Add working solution for building libsndfile DLLs.

* Add libsndfile, g72x, and gsm610 MSVC project files.
* Add a hacked config.h file that includes shim defines for POSIX things.
* Add __declspec(dllexport) to functions in sndfile.h.
* Usual solution settings: v120_xp, DLL, MD/MDd, subsystem windows, output directories, etc.
Add script to build libsndfile.

Also, call vcvarsall.bat from build_environment.bat to fix an msbuild issue with 64-bit compilation in the libsndfile solution.