Detect homebrew OpenSSL on OSX (was:Fix compilation on macOS)
On macOS the build toolchain is broken in the latest master (commit://4a3b61a5b32d4df7d627f993deec97f1db451910).
This is due to the fact that macOS comes with an ancient version of openssl that the tor autoconf script will resolve to first. Instead, if the user has installed homebrew, it should first pick the homebrew version.
In this branch I added a patch to the autoconf script that fixes this: https://github.com/hellais/tor/commit/e55a198c517bb5e40acd395e562891c4927e54a9.
More details about the fix are in the commit message and comment in the code.
I tested that this works as expected on macOS 10.12.6 with homebrew 1.3.2 and openssl 1.0.2l.