Support for OpenBSD

- use fseeko and ftello like FreeBSD in tinyxml2
- use current directory as basePath if NULL (OpenBSD doesn't actually support this feature it is disabled via a patch in their ports)

desktop_version/CONTRIBUTORS.txt

@@ -25,6 +25,7 @@ Contributors
 * Thomas Sänger (@HorayNarea)
 * Info Teddy (@InfoTeddy)
 * Pierre-Alain TORET (@daftaupe)
+* Jules de Sartiges (@strikersh)
 * leo60228 (@leo60228)
 * Emmanuel Vadot (@evadot)
 * Rémi Verschelde (@akien-mga)

desktop_version/src/FileSystemUtils.cpp

@@ -148,8 +148,8 @@ int FILESYSTEM_init(char *argvZero, char* baseDir, char *assetsPath)
 
 	if (basePath == NULL)
 	{
-		puts("Unable to get base path!");
-		return 0;
+		puts("Unable to determine base path, falling back to current directory");
+		basePath = SDL_strdup("./");
 	}
 
 	/* Mount the stock content last */