In particular, the implementation of posix._fcopyfile uses <copyfile.h> (available since OS X 10.5), and the implementation of threading.get_native_id uses pthread_threadid_np (available since OS X 10.6). This breaks builds for OS X 10.5 and older.

I'm aware that the oldest officially supported OS X version is 10.6, but according to a python-dev post (https://mail.python.org/pipermail/python-dev/2018-May/153725.html), earlier versions are "supported on a best-effort basis".

Would patches for these old versions still be accepted? I have the patch for this issue almost completely worked out, and it's not very complicated or intrusive - it mainly just adds standard autoconf checks for the functions/headers in question.

For current releases, which only provide 64-bit binaries, 10.9 is the lated supported version.  See 
https://www.python.org/downloads/release/python-377/
for example.  I believe this should be closed.

Thanks for the report. Feel free to submit a PR or patch. I can’t guarantee we will accept it if it’s too intrusive. (The request here is to fix building from source for these older systems not to provide binary releases.)