Don't use '--output /dev/null' for Pandoc test

Under MSYS2 Pandoc redirects this as a request to open the regular
nul: device of Windows and then returns an error because it cannot
find it.

m4/AGS_PROG_PANDOC.m4

@@ -4,7 +4,7 @@ AC_ARG_VAR([PANDOC], [path to PANDOC])
 AS_IF([test "x${PANDOC}" = x],
      [AC_CACHE_CHECK([for pandoc], [ac_cv_path_PANDOC],
          [AC_PATH_PROGS_FEATURE_CHECK([PANDOC], [pandoc],
-             [[echo | "$ac_path_PANDOC" --to "$1" --output /dev/null && \
+             [[echo | "$ac_path_PANDOC" --to "$1" >/dev/null && \
                    ac_cv_path_PANDOC="$ac_path_PANDOC" ac_path_PANDOC_found=:]],
              [AC_MSG_ERROR([could not find pandoc with required features])])])