I looked at the gnulib docs but honestly it looks a bit complicated..
I think that, for the beginning, I will compile the Windows DLL with the mingw-w64 toolchain in an msys2 environment.
I think that should work fine, even from the official MSVC-compiled cpython distribution, because I'm not planning to make a python extension module (i.e. shared object that calls into the Python C API and requires to link with the python library, thus needs to be compiled with the same compiler as python itself), but just a shared library independent from python that is dynamically loaded at runtime via dlopen or LoadLibrary by ctypes or cffi.
This way I can use the normal autotools build system on all three operating systems.
I just need to figure out how to create a shared libttfautohint.so, as the current setup normally only creates a static library embedded in the ttfautohint executable.
If you have any tips, please let me know, thank you!