workaround for double-destruction bugs in compilers: with this, boost::exception objects should survive the case when the destructor is called twice.

[SVN r61602]

libs/exception

@@ -1 +1 @@
-Subproject commit f10defd7a2033bc5f52ce3959d5dc6767dce43dd
+Subproject commit c251404656f01e920ffe343ebeef0fd778ac65ba