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]
requires = [
    "setuptools>=42",
    "setuptools_scm",
    "wheel",
    "cmake>=3.15",
    "pybind11[global]",
]
build-backend = "setuptools.build_meta"