Start building as a shared library on Windows (DLL)

[wravery]

This is a potential fix for Issue #39. It depends on having python2 and winflexbison in the program path because the output of flex and bison is a little different with the --wincompat flag. We can't just use the checked in versions of those files on Windows.

Alternatively, Microsoft/vcpkg can acquire those tools on demand for Windows. I have a pull request to that project (#3953) to add a port for this library, it patches the CMakeList.txt file to add these tools to the path without requiring a separate install on Windows. The same port works on the Linux version of vcpkg as well.

[wravery]

BTW, here's what I built with it: cppgraphqlgen. 😁