If a target's .do file disappears, don't forget to stamp it.

If a file previously was generated but now isn't (ie. its .do file
disappears), we would never re-stamp that target, and so all its
dependencies would rebuild continually.

builder.py

@@ -70,6 +70,7 @@ class BuildJob:
         (dofile, basename, ext) = _find_do_file(t)
         if not dofile:
             if os.path.exists(t):
+                state.stamp(t)
                 return self._after2(0)
             else:
                 err('no rule to make %r\n' % t)