Provide missing description for our FLEXIBLE_ARRAY_MEMBER defines

Apparently autoheader throws a tantrum if you say 'AC_DEFINE([a],
[b])'.  Instead you must say 'AC_DEFINE([a], [b], [description of
a])'.  We were running into this in our replacement definitions for
FLEXIBLE_ARRAY_MEMBER, which were only getting built on autoconf
versions before 2.61 -- and this made us stop working with those
autoconf versinos.

Fixes bug 2430; bugfix on 0.2.3.1-alpha.

changes/bug2430

+  o Minor bugfixes:
+    - Fix a bug in configure.in that kept it from building a configure
+      script with autoconf versions earlier than 2.61. Fixes bug 2430;
+      bugfix on 0.2.3.1-alpha.

configure.in

@@ -189,9 +189,9 @@ AC_C_FLEXIBLE_ARRAY_MEMBER
   [tor_cv_c_flexarray=yes],
   [tor_cv_c_flexarray=no])])
  if test $tor_cv_flexarray = yes ; then
-   AC_DEFINE([FLEXIBLE_ARRAY_MEMBER], [])
+   AC_DEFINE([FLEXIBLE_ARRAY_MEMBER], [], [Define to nothing if C supports flexible array members, and to 1 if it does not.])
  else
-   AC_DEFINE([FLEXIBLE_ARRAY_MEMBER], [1])
+   AC_DEFINE([FLEXIBLE_ARRAY_MEMBER], [1], [Define to nothing if C supports flexible array members, and to 1 if it does not.])
  fi
 ])