[RFC] Don't overwrite an existing Makefile

Hi, Simon - Another problem with "Setup makefile" is that it will overwrite an existing Makefile, even if that file perhaps ought to be present. For example, using "Setup makefile" in a Cabal repo clobbers Cabal's own Makefile. The attached patch causes "Setup makefile" to refuse to overwrite an existing file. I'm a bit ambivalent about it, but I don't see another simple, predictable way to avoid the possibility for foot-shooting.