1. When qhull is available system-wide but its version cannot be
   determined, ignore it (there is no guarantee that it will, in fact,
   work) and fallback to the vendored version.
2. When adding header paths to vendored libraries, prepend the paths to
   give them higher priority than /usr/{,local/}include. (In reality we
   should just not add the system /usr/{,local/}include paths to the
   compiler invocations at all, but that's a more complicated patch.

Closes #8390, closes #8872; partial fix for #9737 (though not actually sufficient for the issue described in #9737).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way