Deal with compile errors caused by enable_shared_from_this reversions.

Added check to make sure postconstructor_invoker doesn't call
adl_postconstruct multiple times.



[SVN r51610]

libs/signals2

@@ -1 +1 @@
-Subproject commit d44d7caed1324906013c54a5c635197287481d9f
+Subproject commit a67605599fa4b2d860b6ba22d612b5065fe69372