Fix: Compile warnings when asserts are disabled

src/gfxinit.cpp

@@ -105,6 +105,7 @@ static void LoadGrfFileIndexed(const char *filename, const SpriteID *index_tbl,
 
 		do {
 			bool b = LoadNextSprite(start, file_index, sprite_id, container_ver);
+			(void)b; // Unused without asserts
 			assert(b);
 			sprite_id++;
 		} while (++start <= end);

src/settings.cpp

@@ -2053,6 +2053,7 @@ uint GetCompanySettingIndex(const char *name)
 {
 	uint i;
 	const SettingDesc *sd = GetSettingFromName(name, &i);
+	(void)sd; // Unused without asserts
 	assert(sd != nullptr && (sd->desc.flags & SGF_PER_COMPANY) != 0);
 	return i;
 }
@@ -2159,6 +2160,7 @@ void IConsoleSetSetting(const char *name, int value)
 {
 	uint index;
 	const SettingDesc *sd = GetSettingFromName(name, &index);
+	(void)sd; // Unused without asserts
 	assert(sd != nullptr);
 	SetSettingValue(index, value);
 }

src/window.cpp

@@ -1386,6 +1386,7 @@ static void AddWindowToZOrdering(Window *w)
 		/* Search down the z-ordering for its location. */
 		Window *v = _z_front_window;
 		uint last_z_priority = UINT_MAX;
+		(void)last_z_priority; // Unused without asserts
 		while (v != nullptr && (v->window_class == WC_INVALID || GetWindowZPriority(v->window_class) > GetWindowZPriority(w->window_class))) {
 			if (v->window_class != WC_INVALID) {
 				/* Sanity check z-ordering, while we're at it. */