The latest release of ohphoneX ships with a binary version of libavcodec.so which is part of the ffmpeg project. The binary is built from a patched version of the ffmpeg source. More information on this patch and on using the H.263 Codec with Openh323 can be found on Guilhem Tardy's website and in the Vox Gratia H.263 Codec document.
The ffmpeg project is released under the GNU LGPL. This permits binary distribution as long as the corresponding source code is also available from the website where you download ohphoneX. That is why you can download the patched ffmpeg source from here.
Please note that when you use this patched version, only the libavcodec part will compile without error. However, this library is also the only one you need for ohphoneX, so please don't ask us if you want to make the libavformat library compile.