bin/60921: m4 --undefine argument does not undefine

["phil%hagelb.org@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60921
>Category:       bin
>Synopsis:       m4 --undefine argument does not undefine
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Oct 24 16:20:00 +0000 2026
>Originator:     Phil Hagelberg
>Release:        11.0
>Organization:
>Environment:
NetBSD coronacht 11.0 NetBSD 11.0 (GENERIC) #0: Thu Jul 30 15:23:12 UTC 2026  mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/amd64/compile/GENERIC amd64
>Description:
According to the man page, m4 accepts a -U or --undefine argument which will take whatever symbol it's given and remove whatever definition is attached to it. Unfortunately, it does not do that.
>How-To-Repeat:
If you run this:

echo "feeling very dnl today" | m4

As you might expect it just prints "feeling very " because dnl discards the rest of the line.

However, you should be able to run this:

echo "feeling very dnl today" | m4 --undefine dnl

and have it emit "feeling very dnl today", and indeed other implementations of m4 do this. But NetBSD's m4 does not.
>Fix: