strnlen: Avoid memchr related link error on old obsolete platforms. * modules/memchr-obsolete: New file. * m4/memchr-obsolete.m4: New file. * m4/memchr.m4 (gl_FUNC_MEMCHR): Don't check whether memchr exists if gl_FUNC_MEMCHR_OBSOLETE is not also defined. * modules/memchr (Depends-on): Add memchr-obsolete. * modules/strnlen (Depends-on): Likewise. * doc/posix-functions/memchr.texi: Mention module memchr-obsolete.