CMAKE_xxx vars are now used in the CMake-generated cmake_install.cmake
script while CPACK_xxx equivalent vars are used from within CPack.
CPack is responsible for getting/forwarding definitions of
CPACK_xxxx var corresponding to CMAKE_xxxx when invoking
CMake-generated install scripts.
As a consequence:
CMAKE_ABSOLUTE_DESTINATION_FILES
CMAKE_WARN_ON_ABSOLUTE_INSTALL_DESTINATION
CMAKE_ERROR_ON_ABSOLUTE_INSTALL_DESTINATION
may be used from outside CPack as well.
e.g.
cmake -DCMAKE_ERROR_ON_ABSOLUTE_INSTALL_DESTINATION=1 -P cmake_install.cmake
works as expected.