tests: be robust to POSIXLY_CORRECT being defined.

* tests/local.at (AT_BISON_CHECK_NO_XML): Check if
POSIXLY_CORRECT is defined, not if it is defined to 1.
Reported by Lie Yan.
http://lists.gnu.org/archive/html/bug-bison/2012-03/msg00000.html

THANKS

@@ -59,6 +59,7 @@ Juan Manuel Guerrero      [email protected]
 Kees Zeelenberg           [email protected]
 Keith Browne              [email protected]
 Laurent Mascherpa         [email protected]
+Lie Yan                   [email protected]
 Magnus Fromreide          [email protected]
 Marc Autret               [email protected]
 Martin Jacobs             [email protected]