In order to compile and install KDiff3 on a system with KDE, type the following in the base directory of the KDiff3 distribution:
kde-dir specifies the directory
containing KDE on your system. If you are not sure, read the README-file for details.
If you don't use KDE don't use configure but follow the instructions for Qt-only systems in the README file.
Since KDiff3 uses autoconf and automake you should have no trouble compiling it. Should you run into problems please report them to the KDE mailing lists.