get_externals.bat downloads and extracts libffi to a versioned directory (much like the other external libraries). See https://github.com/python/cpython/blob/v3.8.0/PCbuild/get_externals.bat#L55
However the binary release is downloaded to an unversioned directory. See https://github.com/python/cpython/blob/v3.8.0/PCbuild/get_externals.bat#L79
The visual studio project looks for the unversioned directory (https://github.com/python/cpython/blob/v3.8.0/PCbuild/python.props#L62), and so the binaries are always used.

So it is possible to build from source, but you have to move libffi yourself after running `get_externals_bat --libffi-src`. I think the fix here is to make get_externals.bat and visual studio always use a versioned directory.

The intent is that you'd build from source and then copy the binaries into the libffi directory (the *real* intent is that very few people should have to build libffi from source).

Can you tell us a bit more about what you were trying to achieve when you ran into this?

Ah you're right. I was trying to install in the same way as previous versions. I didn't realise that calling prepare_*.bat was a separate step now.
Sorry about that.