I looked at what is required to build tftp in C99 mode. The issue is that autoreconf ends up defining _GNU_SOURCE, and due to the way <signal.h> in glibc works today, that makes bsd_signal unavailable. This results in an implicit function declaration in various source files, and future compilers will reject that by default with an error.

I proposed to change that on the glibc side:

[PATCH] <signal.h>: Make bsd_signal available with _GNU_SOURCE
<https://sourceware.org/pipermail/libc-alpha/2022-November/143616.html>

If that doesn't work out, we may have to patch tftp to call the signal function instead of bsd_signal. They are equivalent for glibc.