Some fixes to is_convertible to get rid of the g++ warning.

This included:
  - changes to is_convertible under g++ #ifdef
  - added parenthesis around body of BOOST_IS_CLASS macro
  - a specialization of is_array for const arrays


[SVN r8822]

libs/type_traits

@@ -1 +1 @@
-Subproject commit de81a215973ffa5eda4b425392011873e55c51ae
+Subproject commit a9d7c5339492888c7669a7dc1b04c638b7ef1d52