boost176 @1.76.0_10 broken after rev-update for new version of icu (on older macOS?)

[snowflake (Dave Evans)]

Lots have ports have received a rev-update after the new version of icu. Most seem to be building OK, but boost176 has some errors, at least on my macOS 10.11.6

Here's one error

...failed darwin.compile.c++ bin.v2/libs/log/build/darwin-16.0.6/release/threadapi-pthread/threading-multi/visibility-hidden/core.o...
darwin.compile.c++ bin.v2/libs/log/build/darwin-16.0.6/release/threadapi-pthread/threading-multi/visibility-hidden/severity_level.o
    "/opt/local/bin/clang++-mp-16"  -ftemplate-depth-1024 -fvisibility-inlines-hidden -Os -std=gnu++11 -stdlib=libc++ -arch x86_64 -stdlib=libc++ -fPIC -m64 -O3 -Wall -fvisibility=hidden -dynamic -gdwarf-2 -fexceptions -Wno-inline  -DBOOST_ALL_NO_LIB=1 -DBOOST_ATOMIC_DYN_LINK=1 -DBOOST_CHRONO_DYN_LINK=1 -DBOOST_FILESYSTEM_DYN_LINK=1 -DBOOST_HAS_ICU=1 -DBOOST_LOG_BUILDING_THE_LIB=1 -DBOOST_LOG_DLL -DBOOST_LOG_USE_NATIVE_SYSLOG -DBOOST_LOG_USE_SSSE3 -DBOOST_LOG_WITHOUT_DEBUG_OUTPUT -DBOOST_LOG_WITHOUT_EVENT_LOG -DBOOST_SPIRIT_USE_PHOENIX_V3=1 -DBOOST_THREAD_BUILD_DLL=1 -DBOOST_THREAD_DONT_USE_CHRONO=1 -DBOOST_THREAD_POSIX -DBOOST_THREAD_USE_DLL=1 -DNDEBUG -D__STDC_CONSTANT_MACROS  -I"." -I"/opt/local/include" -I"libs/log/src"  -c -o "bin.v2/libs/log/build/darwin-16.0.6/release/threadapi-pthread/threading-multi/visibility-hidden/severity_level.o" "libs/log/src/severity_level.cpp"
In file included from libs/log/src/severity_level.cpp:18:
In file included from ./boost/log/sources/severity_feature.hpp:31:
In file included from ./boost/log/utility/strictest_lock.hpp:19:
In file included from ./boost/mpl/integral_c.hpp:32:
./boost/mpl/aux_/integral_wrapper.hpp:73:31: error: integer value -1 is outside the valid range of values [0, 3] for this enumeration type [-Wenum-constexpr-conversion]
    typedef AUX_WRAPPER_INST( BOOST_MPL_AUX_STATIC_CAST(AUX_WRAPPER_VALUE_TYPE, (value - 1)) ) prior;
                              ^
./boost/mpl/aux_/static_cast.hpp:24:47: note: expanded from macro 'BOOST_MPL_AUX_STATIC_CAST'
#   define BOOST_MPL_AUX_STATIC_CAST(T, expr) static_cast<T>(expr)
                                              ^
1 error generated.