I've started a build script specifically for Mac OSX.
https://github.com/kalebheitzman/macosx ... flightgear
run ./build.sh from the command line to use it. It'll create an FGFS directory in your Home path, aka /Users/Kaleb etc. I'll be developing it more over the next couple of weeks but at the moment it builds a successful install of simgear, flightgear, and fgx.
I may have had some dependencies installed on my computer already using Homebrew so if I've missed something in the script let me know.