Starting with KDE 4.0.0 source compatibility is guaranteed, this doesn't apply only to the C++ sources, but also to the CMake interface.
At least version 2.6.2 of CMake is required now. With KDE 4.0 and 4.1 version 2.4.5 was required.
The option KDE4_ENABLE_EXPERIMENTAL_LIB_EXPORT was new in KDE 4.1 and enabled the reduced link interface. This is now always the case, so the option became unnecessary.
KDE4_USE_ALWAYS_FULL_RPATH was by default on. In this case executables, libraries and plugins where linked with RPATH. This caused relinking of these binaries during make install, which takes some time. If it was off, only executables where linked with RPATH, and their RPATH had to be sufficient also for the shared libraries and plugins. This can be the case, but doesn't have to. Applications which were not marked as RUN_UNINSTALLED where built directly with their install RPATH. This saved a lot of time during make install but the resulting install might not get the correct libraries. Since version 2.6 CMake can now edit the RPATH inside binaries, which compared to relinking takes no time. That's why this option is not necessary anymore, and now all executables, libraries and plugins are always built with their build tree RPATH and then installed with their install tree RPATH, so they always get their correct libraries, both when executed from the build tree as well as when executed from the install tree.
With KDE 4.2 and the required CMake 2.6.2 the FIND_PACKAGE() command now supports finding installed configuration files of packages. If the package is named <package>, their configuration file is named <package>Config.cmake. CMake 2.6.2 supports searching these files in lib/<package>/cmake/, a version number may also be included in the directory name: lib/<package>-x.y.z/cmake/. Starting with version 2.6.3 of CMake (soon to be released as of January 2009), it also supports searching these configuration files in lib/cmake/<package>/ (also with optional version number). This may be preferred by packagers. If a KDE package installs such a configuration file it should check this option, and if enabled install its <package>Config.cmake file to lib/cmake/<package>/ instead of lib/<package>/cmake/.
This option is only available with CMake >= 2.6.3, in this case it is ON by default. Otherwise it is OFF.
If a package has been installed with this option enabled (which can only be the case with CMake >= 2.6.3), at least CMake 2.6.3 is also required for building packages which use this package. CMake < 2.6.3 will not find their installed <package>Config.cmake files.
Currently only kdepimlibs/ and kdebase/workspace/ install such configuration files.