Summary:
Spin around our open/closed source checks. Previously we defined `OSS_ENABLE` in open source builds. This change defines `OI_META_INTERNAL` instead. This is nicer, as external users don't have to do anything special to get a working build.
Use this new macro to define a boolean constant in a new header `Portability.h`. This is inspired by Folly, and makes the internal build easier - definitions in Buck2 have to propagate up from a dependency instead of down from one. Annoyingly we can't use `if constexpr` for a lot of the previous `#ifdef` blocks as we conditionally include the headers. Longer term we could fix this by exposing a header interface but no source, allowing these to build but not be compiled in. For now I did something weird: I defined a function style macro in `Portability.h` based on the compile time macro. This forces you to have included `Portability.h` before using it to ensure the definition is everywhere. Open to feedback as I haven't seen anyone else do this.
Reviewed By: tyroguru
Differential Revision: D50000454
