verify-header-sets
------------------

* A new :prop_tgt:`VERIFY_HEADER_SETS` target property was added, which can be
  used to verify that all headers in header sets can be used on their own.
* A new :variable:`CMAKE_VERIFY_HEADER_SETS` variable was added, which is used
  to initialize the :prop_tgt:`VERIFY_HEADER_SETS` target property.