I noticed a while ago that the CMake installation variable names used by KDE projects (defined either in kdelibs or KDEInstallDirs in extra-cmake-modules) are inconsistent with what GNUInstallDirs, for example, uses. The traditional variable names are things like BIN_INSTALL_DIR and CMAKECONFIG_INSTALL_PREFIX, while GNUInstallDirs defines things like CMAKE_INSTALL_BINDIR and CMAKE_INSTALL_DOCDIR.
In an effort to make the installation variables more consistent and compatible with GNUInstallDirs (for the ease of packagers and developers alike), I introduced GNUInstallDirs-style variables to KDEInstallDirs – this included the variables defined by GNUInstallDirs and other ones beginning CMAKE_INSTALL_. These were kept in sync with the old-style variable names.
Both of these styles of variables have namespacing problems: the ones starting CMAKE_ tread on CMake’s namespace (so CMake could potentially define one of those variables that doesn’t appear in GNUInstallDirs to have a different meaning), and the old-style variables are very generic and could conflict with another CMake module. Neither of these are likely to be an issue in reality, but it is good practice to respect namespaces in third-party modules.
KDEInstallDirs now primarily defines variables starting with KDE_INSTALL_ (which corresponds with the module name). The old-style names and all the CMAKE_INSTALL_ names that do not also exist in GNUInstallDirs are deprecated, and can be disabled entirely (see the documentation). It is possible to disable the variables that appear in GNUInstallDirs as well.
This appeared in extra-cmake-modules 1.6.0, but that version had some flaws (for example, it would break if you used both GNUInstallDirs and KDEInstallDirs in the same project, which was a major oversight), so you should use 1.6.1 instead.
There is a script in the kf5 directory of the kde-dev-scripts.git repository to port your CMakeLists.txt files to the new variables: cmakelists_install_vars.pl. Just run it in a directory with a CMakeLists.txt file.