Summary:
We are seeing random segment fault originating from OpenSSL on macOS when
Mononoke fetching is enabled.

The cause is that on macOS we are actually linking against libcurl shipped with
the system instead of ours. That copy of libcurl is linked with macOS's
libcrypto instead of the one we compiles during Eden's build, and it seems that
version of libcrypto does not provide concurrency safety.

The solution is to build curl on macOS and make sure it is linked to our
OpenSSL that has the concurrency callbacks registered.

Reviewed By: wez

Differential Revision: D17657822

fbshipit-source-id: 85abdf3be10b3903a5efc6b3a91624c7258de790