AC_CHECK_HEADER_STDBOOL regression with trunk autoconf

[Todd C. Miller]

Commit 6dcecb780a69bd208088d666b299e92aa7ae7e80 "Port
AC_CHECK_HEADER_STDBOOL to C23" causes AC_CHECK_HEADER_STDBOOL to
fail even on systems with a conforming stdbool.h.

There is no longer an 'a' variable so it should not be referenced
in the return statement.

 - todd

diff --git a/lib/autoconf/headers.m4 b/lib/autoconf/headers.m4
index 5fde7f5a..43f55ed2 100644
--- a/lib/autoconf/headers.m4
+++ b/lib/autoconf/headers.m4
@@ -639,7 +639,7 @@ AC_DEFUN([AC_CHECK_HEADER_STDBOOL],
 
              /* Refer to every declared value, so they cannot be
                 discarded as unused.  */
-             return (!a + !b + !c + !d + !e + !f + !g + !h + !i + !j + !k
+             return (!b + !c + !d + !e + !f + !g + !h + !i + !j + !k
                      + !l + !m + !n + !o + !p + !pp + !ps);
            ]])],
         [ac_cv_header_stdbool_h=yes],