incorrect build cancel procedure on Windows

Cancelling the C++ build kills the toplevel process immediately while child processes keep running. The result is that the compilers and linkers keep running due to multi-processor build.

Killing all child processes immediately wouldn't be useful either. Visual Studio doesn't use temporary files, writing immediately to outputs. Killing the build tool's children would also mess up .ipdb files leading to ICE's on next compiler invocation, as well as keeping broken binary object/output files.

The solution is to kill as-if sending a Control-C. Sending that from a terminal session does the right thing when sent to the Ninja build tool.