SoF S4: Improved the reminder to recruit miners (#3422) (fixes #3394)

Updated to [allow_undo], because the hint message shouldn't block undoing a
recall.  Recruiting can't be undone, even with [allow_undo], so this isn't
allowing a cheater's opportunity to recruit units with desired traits.

(cherry-picked from commit 66c56403db)

data/campaigns/Sceptre_of_Fire/scenarios/4_Gathering_Materials.cfg

@@ -571,8 +571,11 @@
         [/message]
     [/event]
 
+    # This reminder will be triggered either on the third turn, or when the
+    # player is running out of gold for recruitment, whichever happens first.
+    # The message will only be displayed once, because it's first_time_only=yes.
     [event]
-        name=turn 3
+        name=reminder_to_recruit_miners
 
         [filter_condition]
             [not]
@@ -588,6 +591,42 @@
         [/message]
     [/event]
 
+    [event]
+        name=turn 3
+
+        [fire_event]
+            name=reminder_to_recruit_miners
+        [/fire_event]
+    [/event]
+    [event]
+        name=recruit,recall
+        first_time_only=no
+
+        [filter]
+            side=1
+        [/filter]
+
+        [store_gold]
+            side=1
+            variable=money_for_recruitment
+        [/store_gold]
+        [if]
+            [variable]
+                name=money_for_recruitment
+                less_than=65
+                # Two miners cost 38, the most expensive recruit is 24, and
+                # upkeep will cost a bit, hence warn around about 65.
+            [/variable]
+            [then]
+                [fire_event]
+                    name=reminder_to_recruit_miners
+                [/fire_event]
+            [/then]
+        [/if]
+        {CLEAR_VARIABLE money_for_recruitment}
+        [allow_undo][/allow_undo]
+    [/event]
+
     [event]
         name=moveto