Add gcc's -Wundef to test suite and fix another warning from it.

Add gcc's -Wundef to test suite and fix another warning from it.

* NEWS (2.4.2): Update description of -Wundef fix.
* configure.ac (WARN_CXXFLAGS_TEST): New substitution.
(WARN_CFLAGS_TEST): New substitution.
* data/glr.c: Avoid warning about __STRICT_ANSI__.
* tests/atlocal.in (CFLAGS): Use WARN_CFLAGS_TEST instead of
WARN_CFLAGS.
(NO_WERROR_CFLAGS): Likewise.
(CXXFLAGS): Use WARN_CXXFLAGS_TEST instead of WARN_CXXFLAGS.
(cherry picked from commit a603c6e0bb8b9e79d4136ee0fd0d1bf6a7d010ef)