libffi: build DLL on Windows

The build setup is rather horrible, with some minimal gcc MSVC
wrapper. But the DLL is a prerequisite for the Python 3.8 build,
which dropped the internal libffi.

It's also possible to build it statically, but then you have to
patch the Python 3 _ctypes msbuild properties.

This also defaults to explicit --build and --host settings, even
without a cross build, because the predicted name would otherwise
differ (*-unknown-* instead of *-pc-*).

Additionally a "make install" also fails...

Change-Id: Ifb7dac840e23efffb9a5e342560aef9e11e0db79
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/98436
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>

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