Better cope with a failed or unavailable ftruncate().

Added HAVE_FTRUNCATE to all the static config-*.h files on the assumption
that all those systems provide it.

src/main.c

@@ -3771,10 +3771,12 @@ operate(struct Configurable *config, int argc, char *argv[])
                           " bytes\n", outs.bytes);
                 fflush(outs.stream);
                 /* truncate file at the position where we started appending */
+#ifdef HAVE_FTRUNCATE
                 ftruncate( fileno(outs.stream), outs.init);
+#endif
                 /* now seek to the end of the file, the position where we
                    just truncated the file */
-                fseek(outs.stream, 0, SEEK_END);
+                fseek(outs.stream, outs.init, SEEK_SET);
                 outs.bytes = 0; /* clear for next round */
               }
               continue;