Add flag to enable building Hermes from source on iOS
Add flag to enable building Hermes from source on iOS

Summary:
If Hermes is enabled on a project and the envvar `BUILD_HERMES_SOURCE` is truthy, a React Native project will build Hermes from source when running `pod install`:

```
cd ios/
BUILD_HERMES_SOURCE=1 pod install
```

* Requires `cmake` and `ninja` to be installed: `brew install cmake ninja`.
* Requires a `react-native` release with Hermes source code bundled in (`scripts/publish-npm.js --include-hermes && npm pack`).

# Changelog

[Internal]

Reviewed By: cortinico

Differential Revision: D34313581

fbshipit-source-id: 4ba5f82aa6c7027fd37f1982127fc5f63d931ded

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