[CMake] ExternalProject_Add always try to connect to SVN server

[David Cole]

If you are using SVN_REVISION with a specific revision number like 
that, you can do:

    UPDATE_COMMAND ""

because there is never any need to update after the initial checkout at 
that revision.

If you then later change to another revision, the parameters to the svn 
checkout ("dowload") step change when you change the revision number, 
and it should do another checkout on a subsequent build after that.


HTH,
David C.