boost/signals2/detail/variadic_slot_invoker.hpp has #include of another boost file with incorrect path (boost/tuple.hpp instead of boost/tuple/tuple.hpp)
% pkg info -l boost-libs | grep boost/tuple.hpp
% pkg info -l boost-libs | grep boost/tuple/tuple.hpp
which prevents consumers of this file from compiling. This was fixed in boost SVN rev 78502, fix it here until boost port is updated to more recent version.
Port maintainer (office@FreeBSD.org) is cc'd.
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new boost is in, and looks fixed.