[PR] Fix homebrew build
[PR] Fix homebrew build

Summary:
`make -j` runs stuff in parallel if their deps don't overlap. ocamlbuild creates the `_build` directory and its subdirectories, with a broken implementation of `mkdir -p`. So, if it runs in parallel, sometimes another process races to create the directory and it crashes.

While I was here, I also updated `all-homebrew` to use a local switch and bare opam repo, to avoid unnecessarily compiling ocaml twice.

Fixes https://github.com/facebook/flow/issues/8027
Pull Request resolved: https://github.com/facebook/flow/pull/8046

Reviewed By: gabelevi

Differential Revision: D17051343

Pulled By: mroch

fbshipit-source-id: 7f68da8016473d04b7dd126b9a2b79de724df6d3

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow