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 getopt during
the bootstrap build, since it assumes the return value is always
a mutable pointer.
Since the pointed-to value is never modified, fix this by making
the pointer const.
MFC after: 1 week
Sponsored by: The FreeBSD Foundation