Dependencies for C++ files in rt are wrong

[nikomatsakis]

I am not 100% sure why, but the autogeneration of dependencies for the C++ files in rt definitely do not work. Changing a header file, for example, does not cause all the files that include that header to be recompiled. I would be inclined to just have *.cpp depend on *.h, but if someone who understands how the dependency generation works wants to try and tackle the problem a smarter way, that's fine with me too. :)