Fix test for data extraction on non Windows system

It was not able to find the file because of the backslash notation.
Use slash notation on non Windows systems to fix it.

gameheaders/stratagus-game-launcher.h

@@ -103,14 +103,6 @@
  * Path to stratagus executable binary
  **/
 
-/**
- * \def TITLE_PNG
- * OPTIONAL: Path to title screen (for testing if data was extracted)
- **/
-#ifndef TITLE_PNG
-#define TITLE_PNG "%s\\graphics\\ui\\title.png"
-#endif
-
 /* Fake definitions for Doxygen */
 #ifdef DOXYGEN
 #define GAME_NAME
@@ -129,6 +121,18 @@
 #define WIN32
 #endif
 
+/**
+ * \def TITLE_PNG
+ * OPTIONAL: Path to title screen (for testing if data was extracted)
+ **/
+#ifndef TITLE_PNG
+#ifdef WIN32
+#define TITLE_PNG "%s\\graphics\\ui\\title.png"
+#else
+#define TITLE_PNG "%s/graphics/ui/title.png"
+#endif
+#endif
+
 #ifndef WIN32
 #if ! defined (DATA_PATH) || ! defined (SCRIPTS_PATH) || ! defined (STRATAGUS_BIN)
 #error You need to define paths, see stratagus-game-launcher.h