Add thread-local storage declaration for VC++

just a very simple #ifdef for VC++

With this change, I can cross-compile in MinGW and then use the DLL to create a LIB, link the LIB and use flint from C++/CLI for easy access from the .net framework.

I still need to copy `sys/param.h` from MinGW imports, I still have to define `_WIN64` and `__MINGW64__`, but at least I can use the headers to access flint.