Detect signed size_t and report an error at configure time.

changes/bug2337

@@ -0,0 +1,3 @@
+  o Minor bugfixes
+    - Detect broken platforms with a signed size_t, and refuse to
+      build there.  Found and analyzed by doorss and rransom.

configure.in

@@ -548,6 +548,23 @@ if test "$tor_cv_time_t_signed" != no; then
             [Define to 1 iff time_t is signed])
 fi
 
+AC_CACHE_CHECK([whether size_t is signed], tor_cv_size_t_signed, [
+AC_RUN_IFELSE(AC_LANG_SOURCE([
+#ifdef HAVE_SYS_TYPES_H
+#include <sys/types.h>
+#endif
+int main(int c, char**v) { if (((size_t)-1)<0) return 1; else return 0; }]),
+  tor_cv_size_t_signed=no, tor_cv_size_t_signed=yes, tor_cv_size_t_signed=cross)
+])
+
+if test "$tor_cv_size_t_signed" = cross; then
+  AC_MSG_NOTICE([Cross compiling: assuming that size_t is not signed.])
+fi
+
+if test "$tor_cv_size_t_signed" = yes; then
+  AC_MSG_ERROR([You have a signed size_t; that's grossly nonconformant.])
+fi
+
 AC_CHECK_SIZEOF(socklen_t, , [AC_INCLUDES_DEFAULT()
 #ifdef HAVE_SYS_SOCKET_H
 #include <sys/socket.h>