Allow to build with the PyPI package "clang" instead of "libclang". 1. It seems to be maintained. 2. In the FreeBSD base system there is no pre-built libclang.so. If you need this library you have to install llvm from ports additionally. 2. On FreeBSD there is no pre-built wheel "libclang" with a packaged libclang.so.

[build-system]
# We define required packages in setup.py:get_requires_for_build_wheel().
requires = []

# See pep-517.
#
build-backend = "setup"
backend-path = ["."]