Sharpened check for 64-bit Unix when defining fseek/ftell macros.

This fixes the issues with VxWorks, as mentioned by @razr in #786.

tinyxml2.cpp

@@ -106,7 +106,7 @@ distribution.
 #elif defined(__APPLE__)
 	#define TIXML_FSEEK fseeko
 	#define TIXML_FTELL ftello
-#elif defined(__x86_64__)
+#elif defined(__unix__) && defined(__x86_64__)
 	#define TIXML_FSEEK fseeko64
 	#define TIXML_FTELL ftello64
 #else