The latest upstream libffi is getting bit-rotten with the Windows/MSVC
builds, and since we are eventually moving to a Meson-based build system,
make use of the Centricular fork of libffi[1], which is better maintained
and tested for builds on Windows, with the use of DLLs over static libraries
and elimination of awkward build flags that we had to use for GObject.
[1]: https://github.com/centricular/libffi
It was suggested that the project files be moved here as we don't actually
need to go two directory layers from $(srcroot), and would help us to
standardize on things in the future across the board.
