[CMake] CMAKE_SYSTEM not available

[Dieter Oberkofler]

I seem unable to get/use the CMAKE_SYSTEM variable using CMake 2.6.4 on the
Windows and Apple platform.
MESSAGE("CMAKE_SYSTEM=(" ${CMAKE_SYSTEM} ")") just shows an empty string
similar as does also CMAKE_SYSTEM_NAME, CMAKE_SYSTEM_VERSION and
CMAKE_SYSTEM_PROCESSOR.
Any ideas on what be the cause for this?
Thank you
-Dieter