As noted recently, OS X comes with OpenSSL 0.9.8 by default, which is too old to build Tor.
During the Tor patch workshop today, rl1987 and Yawning were testing the OpenSSL and Tor builds on OS X.
I've put the OS X build steps they used on the Tor Wiki, along with the alternative method of obtaining OpenSSL and/or Tor via MacPorts:
If you're building Tor on OS X, please try these instructions, fix any mistakes, and add missing steps or other alternatives. In particular, HomeBrew instructions for OpenSSL and Tor would be really helpful for some users.