Update osx build instructions to ensure users are building with the correct version of OpenSSL when using HomeBrew

It's possible that once Brew has installed OpenSSL it won't be used when compiling.

This means that the version bundled with OSX will be used, which is currently 0.9.8.r

This adds a note to the build instructions to help users check that the are using the correct version, and if not, how to ensure they do.