Fix builds on Windows (using clang-cl)

[fanc999]

Hi,

This PR attempts to add a few simple fixes to build on Windows using clang-cl, by:

• Ensure that the public symbols are marked with GTHREE_API in the public headers, so that they are exported during the build.
• Define _USE_MATH_DEFINES as we are using _M_PI with the Visual Studio and Windows SDK headers, which are consumed by the clang-clcompiler.

With blessings, thank you!