New name UI_UPDATE for interpret_move's return "".

Now midend.c directly tests the returned pointer for equality to this
value, instead of checking whether it's the empty string.

A minor effect of this is that games may now return a dynamically
allocated empty string from interpret_move() and treat it as just
another legal move description. But I don't expect anyone to be
perverse enough to actually do that! The main purpose is that it
avoids returning a string literal from a function whose return type is
a pointer to _non-const_ char, i.e. we are now one step closer to
being able to make this code base clean under -Wwrite-strings.

1 files changed, 4 insertions, 4 deletions

diff --git a/towers.c b/towers.c
index a3a7e55..06b7279 100644
--- a/towers.c
+++ b/towers.c
@@ -1349,7 +1349,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
                 ui->hpencil = 0;
             }
             ui->hcursor = 0;
-            return "";		       /* UI activity occurred */
+            return UI_UPDATE;
         }
         if (button == RIGHT_BUTTON) {
             /*
@@ -1369,7 +1369,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
                 ui->hshow = 0;
             }
             ui->hcursor = 0;
-            return "";		       /* UI activity occurred */
+            return UI_UPDATE;
         }
     } else if (button == LEFT_BUTTON) {
         if (is_clue(state, tx, ty)) {
@@ -1394,13 +1394,13 @@ static char *interpret_move(const game_state *state, game_ui *ui,
         }
         move_cursor(button, &ui->hx, &ui->hy, w, w, 0);
         ui->hshow = ui->hcursor = 1;
-        return "";
+        return UI_UPDATE;
     }
     if (ui->hshow &&
         (button == CURSOR_SELECT)) {
         ui->hpencil = 1 - ui->hpencil;
         ui->hcursor = 1;
-        return "";
+        return UI_UPDATE;
     }
 
     if (ui->hshow &&