Respect XDG spec for dosbox main .conf file

Windows and macOS config files are left as they were, this change
affects mostly Linux users.

Users are not forced to migrate, but visible warning is being issued if
they aren't. Majority of users probably won't notice it, as the stable
release should generate newly named .conf file.

include/cross.h

@@ -76,6 +76,8 @@
 static inline float powf (float x, float y) { return (float) pow (x,y); }
 #endif
 
+void CROSS_DetermineConfigPaths();
+
 class Cross {
 public:
 	static void GetPlatformConfigDir(std::string& in);