If you want to re-compile the C part of the application you'll need any C compiler that you prefer, the applications are written entirely in ANSI C and I used for development the GNU C compiler and as IDE - KDevelop on Linux, and Bloodshed Dev-C++ on Windows.
The sources are available under 'src/3gl' subdirectory under the application root directory.
On Windows for both the shell application and messenger application I've created projects for Dev-C++ and Microsoft Visual C. You need to link the projects with WinSock2 library - Ws2_32.lib.
On Unix systems you should follow the standard procedure: