wafHook: init

The waf build system is python-based and hosted locally in each package in the executable file named "waf". Unlike CMake, it cannot generate makefiles so we end up having to override the configure, build, and install phases. I've tried to keep these as close to what's in setup.sh as possible. If there is no waf file in the root directory, then we just copy the one hosted in Nixpkgs. Otherwise the only thing you have to add to a package using Waf is "wafHook" into nativeBuildInputs. wafFlags controls the flags specifically passed to waf while configureFlags, buildFlags, and installFlags are still used as in the generic builder.