(svn r4911) -Fix (FS#156): The Ai no longer attempts to build signals under bridges.

NOTE 1: Remove/revert this for the bridge branch
			   NOTE 2: Bug displays fundamental flaw in command system:
			           If two commands, where command 2 depends on command 1, have
			           to be checked, all those checks need to be manual.
			           (like in this example, first build rail then build signal.
			            BuildSignal cannot be checked without DC_EXEC as the rail
			            isn't there yet)

ai/default/default.c

@@ -1587,8 +1587,9 @@ clear_town_stuff:;
 
 			/* signals too? */
 			if (j & 3) {
-				// Can't build signals on a road.
-				if (IsTileType(c, MP_STREET)) return CMD_ERROR;
+				/* XXX - we need to check manually whether we can build a signal if DC_EXEC is
+				   not set because the rail has not actually been built */
+				if (!IsTileType(c, MP_RAILWAY)) return CMD_ERROR;
 
 				if (flag & DC_EXEC) {
 					j = 4 - j;