add a `LIBFFI_LIBS` to `setup.py` allowing to override the autodetection

# Feature or enhancement

Currently the libffi autodetection supports an include override through `LIBFFI_INCLUDEDIR` but overriding the library requires patching `setup.py`

# Pitch

Typical usage would be to link vs `libffi_pic.a` when building an embeddable Python that is expected work across Linux distros 
