Refs #10387. OPENSSL_ROOT_DIR now points to homebrew opt directory

OPENSSL_ROOT_DIR should point to /usr/local/opt/openssl rather than to a specific version in the cellar. This way the build will continue to function after homebrew updates openssl and deletes the older version. 

testing: code review and make sure it still builds on OS X Mavericks + clang

http://trac.mantidproject.org/mantid/ticket/10387