I don't want to devalue your work either but I don't like the idea of conflating building something from source with packaging an extension.
There is often a one to one correspondence between a program with a makefile and an extension but there are a lot of edge cases: no makefile, custom (ie, non-autoconf) configure script, exotic build systems such as jam or scons.
Moreover, there is a need to build nonsoftware extensions like doc extensions, locale extensions, and metaextensions.
That's why I wrote my script to worry only about taking specified data and turning it into an extension and not worrying about how that data got there or where it came from. However, after saying all of that it is still extremely common to have a correspondence between an extension and an autoconf/automake'd tarball so I do think your script is very useful.
I'm open to working together on this. I'd love to make this script bullet proof, but I don't see much point in working the extension part of it into a Makefile. There are a lot of parts that could be skipped if they exist but there's no good way to make sure they're out of sync that'd be quicker than just redoing it all which doesn't take that much time or effort. And though technically make could parallelize large sections of the process, I'd think the overhead of that parallelizing would likely make the process slower, and even if it doesn't the speed up would be infinitesimal in all but the case of mksquashfs which could be easily &'d and wait'd for if that's the case.
Perhaps you could have your Makefile invoke my script for the extension building part so that it's all in one place, useful on its own, but easily augmented by your Makefile in the case that you want to just build, package, and deploy?