Update build-osx.md

Don't recommend people force OpenSSL into their Homebrew prefix. It creates *so so* many build problems for other software, and can cause serious issues for OS X as a whole. This change was accepted by Bitcoin [here](https://github.com/bitcoin/bitcoin/pull/4753). Dogecoin's configure process should already make this line about forcing OpenSSL redundant as well, and does seem to.