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.
diff --git a/tinyxml2.h b/tinyxml2.h
index fb4376d..2dc98d3 100755
--- a/tinyxml2.h
+++ b/tinyxml2.h
@@ -72,6 +72,8 @@
 #   else

 #       define TINYXML2_LIB

 #   endif

+#elif __GNUC__ >= 4

+#   define TINYXML2_LIB __attribute__((visibility("default")))

 #else

 #   define TINYXML2_LIB

 #endif