You can break the pipe and curl the file first, read it, and then run it. But I doubt that anyone ever reads through the thousands of lines of m4 that come with a typical program that uses autoconf either.
If the project uses Autoconf/Automake, then you can just read the .in files instead. If they include anything unexpected, then it will be pretty obvious (since anything unexpected will be a lot more complicated–looking than anything normal). But if they do include a bunch of custom m4 files, then you’re going to be spending more time on it than you would want.