Handle case in _configure_openmp() where MacOS system doesn't have Homebrew

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.

Author: isobelhooper

setup.py

@@ -505,7 +505,7 @@ def _configure_openmp(self):
                         self.opts.append(flag)
                         self.link_opts.extend((l_arg, flag))
                         return
-            except subprocess.CalledProcessError:
+            except (subprocess.CalledProcessError, OSError):
                 pass
 
             try: