I wrote a Makefile that will download all the files, generate the sources, and build a command line util called ilbc-decode. This should work on any system that uses the GNU build tools, so Mac OS X, GNU/Linux, Windows/Cygwin, Windows/MinGW, and the BSDs.

To use, run these lines in the Terminal:
gunzip Makefile.gz
make generate-sources
make
make install
ilbc-decode

Also, if you are on Mac OS X, you can use Fink to install it:
fink install ilbc-decode