Update build-osx.md

The homebrew instructions were outdated - berkeley-db4 hasn't worked for months, based on the questions I'm seeing on Google/SO.  So I added a section explaining how to install berkeley-db4 using homebrew and move on with your life.  Thanks for the rest of the documentation!
Merge pull request #4740

b1ed7c2 Update build-osx.md (Rose Toomey)