m4: Const correctness for C23

On some platforms, e.g. Linux Clang 22.1.8 / glibc 2.43, strchr()
now implements the C23 behaviour where passing a const pointer to
strchr() also returns a const pointer. This breaks m4 during the
bootstrap build, since it assumes the return value is always a
mutable pointer.

Since the returned value is never modified, simply make the
temporary const.

MFC after: 1 week
Reviewed by: bapt, dim
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58494

(cherry picked from commit 1d94e2e0f2ee21d5a4596efc0570d06e3dea0a6e)