If you are not using revision control then you can specify working copy version CMake variables manually when you configure your project, either using the CMake GUI or by adding these arguments when you call cmake.exe:
-DYourExtensionName_WC_REVISION:STRING=100 -DEXTENSION_WC_REVISION:STRING=100
(replace YourExtensionName and 100 with the actual name of your extension and a revision number)