How can I compile Shotwell to be able to use the faces feature?

The Faces feature was #ifdef'd out of the source until Yorba could add a little more functionality to Shotwell to make it truly useful -- see Bugzilla - Upload faces to facebook and Bugzilla - Face detection. However, as we continued developing other features for Shotwell, the Faces code broke, so there's no way to build Shotwell with Faces support from trunk; see http://redmine.yorba.org/issues/4518

As you mentioned, a patch was made available at git://github.com/playya/shotwell.git. When I build and run Shotwell using this branch, I see the Faces feature and it appears to work. Here's how I did it:

$ git clone git://github.com/playya/shotwell.git shotwell-faces
$ cd shotwell-faces
$ ./configure --define=ENABLE_FACES
$ make
$ ./shotwell

It appears you can also use ./configure --enable-faces, but I tried the above way first and it appeared to work.

Note that you can run Shotwell directly from the build directory. However, to make it available on your desktop and integrate with Nautilus and so forth, you'll need to do this:

$ sudo make install

To see the Faces button, open a photo (double-click on it). It's only available in the photo editor.

Warning: This fork has not been reviewed by Yorba yet. We can make no promises when the Faces feature will be fully integrated into Shotwell. There are other risks using the playya fork, including losing the ability to upgrade your database later. I recommend backing up your database before using it: http://redmine.yorba.org/projects/shotwell/wiki/ShotwellFAQ#How-can-I-back-up-my-Shotwell-library