Merge some boostbook changes from trunk. Fixes #5480.

The default/delete support, plus fix/update tests.




[SVN r84302]

tools/boostbook

@@ -1 +1 @@
-Subproject commit d208dbcb2648fa5f744395fbd5084152e025d071
+Subproject commit cfe6f00c5129c566be712c70e4e3e1a9648a8b13