Added BOOST_CONCEPT_USAGE for defining usage requirements: Using this

macro is better than simply defining a destructor because usage
patterns will be tested whenever associated types are accessed.


[SVN r35198]

libs/concept_check

@@ -1 +1 @@
-Subproject commit 6b4d01994b6ccf3c1d92a32b6ce7cf1f7bf5588e
+Subproject commit ac6415fac2eab9c61659fb3962a00b0fa0ff9c47