open-scene-graph: For 10.6 fix patch and use HB clang

Open-scene-graph (osg) uses a `defaults read` command that's
broken on 10.6.  Patching that means both an inline patch
and an inreplace afterwards to insert the OSX version.

Also the existing patch for the missing `OR` may have worked,
but it turns out there's another CMakeLists.txt that needs
to be patched.

Also on 10.6, no default compiler works due to very ugly
errors about templates and overlaoded functions.  However,
osg builds just fine with HB clang-3.1 from the llvm formula.

- Add a Requirement on 10.6 for HB clang-3.1.
- Patch the top level `CMakeLists.txt` for `OSG_OSX_SDK_VERSION`
- `inreplace` the new `OSG_OSX_SDK_VERSION` to set the version.
- Patch two `CMakeLists.txt` files for `OR` syntax errors.

Fixes #16076