Cast void* for MSVC compatibility
Set test environment on Windows

Add directory containing cmark.dll to PATH on Windows.
Run nmake test with /nologo
Fix MSVC options

Also disable some warnings.
Build instructions for MSVC/NMAKE
Add option to select cmake generator

This allows to build under MSYS. The MSYS generator can be selected with

    make GENERATOR="MSYS Makefiles"

But the default "UNIX Makefiles" generator also seems to work.
Merge pull request #218 from nwellnhof/windows_fixes

Windows fixes