Hello gcc devs.

I have ported libgccjit to Windows. I have tested it with the
native-compilation branch of Emacs so I'm confident that it works well.

The work is not finished though, I could use some help with these two
points:

I have had to concede defeat to libtool and Automake. I could not get
libgccjit
to create a dll and put it in the correct directories. So for now we'll
have to
copy lib/libgccjit.so to bin/libgccjit.dll.

It is not necessary to use --enable-host-shared in Windows (I tested it),
but I
don't know the proper way to disable that check.

Nicolas