CMake build should give Libevent a hint to use HomeBrew OpenSSL on OSX

The CMake build currently uses standard CMake modules (i.e. find_package) to find OpenSSL and Libevent installed on the system. On OSX however, CMake will generally find the system installed version of OpenSSL rather than the more up-to-date HomeBrew package.

We should consider giving this hint to the Libevent package finder:

-DOPENSSL_ROOT_DIR=`brew --prefix openssl`