There were two issues on OSX - we need to give gcc the clang
warnings flags because `gcc` *is* clang on OSX and we missed an
`-std=c++11` on one of the clang++ calls, causing compile failures.
This adds a new annotation for objects we take by reference in the
C header indicating the pointers must not be null. We have to
disable some warning clang now dumps that we haven't annotated all
pointers, as cbindgen is not yet able to add a nullable annotation.
Until we get the bindings generation process super stable, let the
bindings get stale with respect to the main repo while still letting
`cargo check` pass.
