[CMake] Unnecessary rebuild of ExternalProject_Add target

[David Cole]

When you give a git repo for ExternalProject, it will *always* execute 
the UPDATE_COMMAND by default. And then, all the steps after update 
will re-execute since update re-ran.

If you want to use a git repo, but prevent this behavior, you can say:

    UPDATE_COMMAND ""

to eliminate the default update step.

You can also snap to a known commit in the repo by using GIT_TAG.


HTH,
David C.