Summary:
- The `bootstrap-osx-homebrew.sh` script is out of date. It previously
  worked when Folly used Autotools, but was not updated when CMake
  replaced Autotools for the build system.
- Update dependencies to install.
- Do not export environment variables when it is not needed; simply pass the
  defines needed directly to the `cmake` invocation.
- Update `README.md` to specify correct dependencies with CMake build
  for both Homebrew and Macports.

Closes #1038
Pull Request resolved: https://github.com/facebook/folly/pull/1041

Reviewed By: yfeldblum

Differential Revision: D14293767

Pulled By: calebmarchent

fbshipit-source-id: 4c53fa105e009090c80690eca8beb5a8c1491a48