Set ELF visibility

Set symbol visibility to "default" on non-Windows platforms. This allows
building with -fvisibility=hidden, which has various advantages, and
which some projects will almost certainly want to do.

tinyxml2.h

@@ -72,6 +72,8 @@ distribution.
 #   else
 #       define TINYXML2_LIB
 #   endif
+#elif __GNUC__ >= 4
+#   define TINYXML2_LIB __attribute__((visibility("default")))
 #else
 #   define TINYXML2_LIB
 #endif