Handle case in _configure_openmp() for MacOS system without Homebrew

[isobelhooper]

On a MacOS system that's not using Homebrew, running brew --prefix llvm with subprocess.check_output doesn't produce a CalledProcessError; instead, when the shell produces a FileNotFoundError it passes it back and check_output raises that instead.

If we check for OSError (the parent of FileNotFoundError, which should handle other OS-related errors that could get passed back) as well, then this should correctly handle this case.

The MacPorts check doesn't have the same issue, because the which command is built in and so which port will raise a CalledProcessError if MacPorts isn't installed.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}