I found the same issue some time ago.
The problem is that it doesn't compile with a recent version of gcc. But it's simple to fix, just adding this to the Makefile to force gcc 9:
CC = gcc-9
But better to apply the patch of lintweaker!
It'd be good to PR or tell the author for a permanent fix.
Yes, but how to contact him? I can't find him on Github, but only on SourceForge, which wants your firstborn's eternal soul if you want to create a login, and without a login you can't send him a message on that platform.
Anyway, they didn't maintain hex2bin after May 2017, so I consider it unmaintained (obviously, since it doesn't build in its current state) and it has a BSD License, so @ericb59, I will send you a merge request within a fortnight. It will make starting out with Fusion-C that much easier.
