Version: Always define CMake_VERSION_IS_DIRTY to 0 or 1

In cmVersionConfig.h we must define this macro with a value because
clients expect it to have one.  Also ensure that all CMakeLists.txt
code paths have an initialized value.

Reported-by: Stephen Kelly <steveire@gmail.com>

Source/CMakeVersionSource.cmake

@@ -24,8 +24,6 @@ if(EXISTS ${CMake_SOURCE_DIR}/.git/HEAD)
         )
       if(dirty)
         set(CMake_VERSION_IS_DIRTY 1)
-      else()
-        set(CMake_VERSION_IS_DIRTY 0)
       endif()
     endif()
   endif()