OpenSSL is a key-only homebrew crate, and doing brew link --force openssl means we

may end up linking against the insecure,
deprecated system OpenSSL while using the headers from Homebrew's openssl.

Instead, we need to explicitly pass the full include/lib paths to the compiler on our OS X builders, which is most easily done via environment variables.

Parts:

• Update Salt code to no longer call brew link --force openssl
• Update Buildbot configuration to pass correct values for the OPENSSL_INCLUDE_DIR and OPENSSL_LIB_DIR environment variables
• Run some command (TODO: which one?) to unlink openssl on our OS X builders

@santagada Can you give me example output for "$(brew --prefix)"? Does this output vary (over time, with openssl package version, randomly, with homebrew updates)? How this varies will inform how we get the right environment variables, due to the property that the Buildbot configuration is executed on the Buildbot master (a Linux machine), not the actual builder. E.g., we may lock down the OpenSSL version and template the string, or we may dynamically get the version, or just wait for the move to NSS/ring/crypto-library-of-your-choice-here.

Companion to servo/servo#13225.