boost.thread exception types are now header-only so some uses of boost.thread can be header only

[SVN r56019]

libs/thread

@@ -1 +1 @@
-Subproject commit 8d22c3869b80651ce2dbe65a5513e081d0f11721
+Subproject commit 47889a8f227723d92cc94cd9e4cbcaa1c4c21f4d