[CMake] Why is ExternalProject trigger-happy about re-downloading source?

[kent williams]

I've been running into this every since we updated to CMake 2.8.4, but
I don't know if it's revision specific.

Sometimes if I change CMakeLists.txt in various directories so that
make will re-run cmake to pickup CMakeLists.txt changes, it triggers
re-downloading VTK & ITK from scratch.

Is using GIT_REPOSITORY dumber than CVS_REPOSITORY & SVN_REPOSITORY?
If I set UPDATE_COMMAND to "", it never re-downloads. But
GIT_REPOSITORY seems to blow away the downloaded source for an
external project and re-download it, even if the CMakeLists.txt change
is unrelated to to the ExternalProject in question.

Is there a way to suppress this behavior?