[cffi is one of those packages that needs a host-side package installed].
This works reasonably well in the current OpenWrt build system, as binaries get built for this package and get installed in the staging-dir `$(STAGING_DIR)/usr/lib/pythonX.Y/site-packages`.
`PKG_BUILD_PARALLEL:=0` is necessary because installing packages with multiple concurrent pip processes can lead to [errors or unexpected results](https://github.com/pypa/pip/issues/2361).