It's mentioned that you need Qt4 4.4.0 or Newer, you have 4.2.1
From the README Requirements and Installation section :
Requirements & Installation
- for KDE4:
From version 0.9.93 on KDiff3 supports KDE4.
For installation on most distributions you usually also require these packages (names as on opensuse):
- g++ (g++, version 3.4 or newer)
- libqt4-devel (Qt4-libs, version 4.4.0 or newer)
- libkde4-devel (KDE4 Header files and development libraries)
- libkonq-devel (optional, needed for the kdiff3-plugin, if not installed then the contextmenu plugin for konqueror won't be built.)
- cmake (>2.6, checks dependencies and creates the Makefiles appopriate for your system)