Adding command-line options after the grammar file name is not
permitted, so disable checks that do that when
maintainer-check-posix is running.
* tests/local.at (AT_BISON_CHECK_NO_XML): Don't run the
problematic checks when POSIXLY_CORRECT=1. Also, for readability,
remove an unnecessary m4_if.
(cherry picked from commit fc7ce9970f)