We need to remove CMakeCache.txt, otherwise cached variables would remain the
old value when they are removed from cmake_vars parameter.
We use `os.remove()` instead of `Node.delete()` because the latter removes the
node instance from its parent's children list. That makes the node be ignored
when storing persistent information after the build, thus the node signature
wouldn't be saved with that approach, which would make waf always think that
the task should be executed.
The default implementation takes into account the task's output and input
nodes. That isn't very well applicable for cmake tasks, so we define proper
uid() methods.
Gustavo Jose de Sousa