The build seems to work well except for a few key differences with earlier Emacs versions.
Compiling a Carbon version for OS X from the /mac directory is no longer possible because that entire directory has been removed. Instead OS X users need to compile a Cocoa version for OS X by following the directions in the /nextstep directory. At first this seems like a major win: reduce old code-bloat and compile with the newer frameworks. But unfortunately this produces an executable that delivers a couple of annoying inconsistencies.
First off, the Global Menu items are no longer in the OSX Menu Bar. It just makes the newer Cocoa version look sloppy and unfinished compared to the Carbon version. The second, and far more annoying difference, is the reservation of the Command-key for OS X-style functions. Command-N (new window), Command-W (close window), Command-C (copy) now all work just like on the Mac. But the infinitely more useful Emacs Meta-key functionality is now relegated to the Alt/Option key. :(