Well, most open-source apps have an existing build system based on gnumake or similar. This isn't really directly compatible with xcode's model for how projects should be built - a lot of its functionality is based off "native" projects where it manages dependencies etc and invokes the compiler itself, without using some other tool.
You can either create an xcode project and use that, or you could just use xcode to edit the source files. You could probably also create a legacy makefile target.