Warn at configure time when time_t is unsigned

Inspired by #8042.

As far as I know, OpenVMS is the only place you're likely to hit an
unsigned time_t these days, and Tor's VMS support
is... lacking. Still worth letting people know about it, though.

changes/warn-unsigned-time_t

@@ -0,0 +1,5 @@
+  o Build improvements:
+    - Warn if building on a platform with an unsigned time_t: there
+      are too many places where Tor currently assumes that time_t can
+      hold negative values. We'd like to fix them all, but probably
+      some will remain.

configure.ac

@@ -972,6 +972,10 @@ AX_CHECK_SIGN([time_t],
 #endif
 ])
 
+if test "$ax_cv_decl_time_t_signed" = no; then
+  AC_MSG_WARN([You have an unsigned time_t; some things will probably break. Please tell the Tor developers about your interesting platform.])
+fi
+
 AX_CHECK_SIGN([size_t],
        [ tor_cv_size_t_signed=yes ],
        [ tor_cv_size_t_signed=no ], [