Look at the right variable when warning about signed size_t.

In 81d69f4c2d8a451 (0.2.21-alpha) we added a compile-time check for
a (totally broken) signed size_t.  In 0e597471af (not yet released)
I switched to a better configure-time check, which stored its output
in a different variable.  I didn't change the code which looked at
the output, however.

This bug is not in any released version of Tor, and would not affect
anybody with a working Tor.

configure.ac

@@ -980,7 +980,7 @@ AX_CHECK_SIGN([size_t],
 #endif
 ])
 
-if test "$tor_cv_size_t_signed" = yes; then
+if test "$ax_cv_decl_size_t_signed" = yes; then
   AC_MSG_ERROR([You have a signed size_t; that's grossly nonconformant.])
 fi