At least cl version 19.14.26433 generates such warning C4996 now.  While the one
about the using declaration in boost/iterator.hpp looks somewhat bogus, the one
in boost/circular_buffer/details.hpp is about an actual use of std::iterator as
a base class in Boost-internal code.  If further such places in external/boost
crop up that need fixing, an alternative approach could be to extend the
external/boost/include/ wrappers to be also used with MSVC.  (Globally setting
_SILENCE_CXX17_ITERATOR_BASE_CLASS_DEPRECATION_WARNING has avoided, as it also
finds uses in our own code that should be fixed, like
58222feb "warning STL4015: std::iterator is
deprecated (clang-cl)".)

Change-Id: I6a41421a590fcee57b4ac8bcb7c3631fdfdecb81
Reviewed-on: https://gerrit.libreoffice.org/58755
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>