sigil-qt4 fails to build with clang: Undefined symbols for architecture x86_64: "boost::scoped_static_mutex_lock::scoped_static_mutex_lock(boost::static_mutex&, bool)"

[barracuda156]

Undefined symbols for architecture x86_64:
  "boost::scoped_static_mutex_lock::scoped_static_mutex_lock(boost::static_mutex&, bool)", referenced from:
      boost::object_cache<boost::re_detail_500::cpp_regex_traits_base<char>, boost::re_detail_500::cpp_regex_traits_implementation<char> >::get(boost::re_detail_500::cpp_regex_traits_base<char> const&, unsigned long) in libFlightCrew.a(Utilities.cpp.o)
      boost::cpp_regex_traits<char>::get_catalog_name() in libFlightCrew.a(Utilities.cpp.o)
      boost::re_detail_500::mem_block_cache::get() in libFlightCrew.a(Utilities.cpp.o)
      boost::re_detail_500::mem_block_cache::put(void*) in libFlightCrew.a(Utilities.cpp.o)
      boost::object_cache<boost::re_detail_500::cpp_regex_traits_base<char>, boost::re_detail_500::cpp_regex_traits_implementation<char> >::get(boost::re_detail_500::cpp_regex_traits_base<char> const&, unsigned long) in libFlightCrew.a(DetermineMimetype.cpp.o)
      boost::cpp_regex_traits<char>::get_catalog_name() in libFlightCrew.a(DetermineMimetype.cpp.o)
      boost::re_detail_500::mem_block_cache::get() in libFlightCrew.a(DetermineMimetype.cpp.o)
      ...
  "boost::scoped_static_mutex_lock::~scoped_static_mutex_lock()", referenced from:
      boost::object_cache<boost::re_detail_500::cpp_regex_traits_base<char>, boost::re_detail_500::cpp_regex_traits_implementation<char> >::get(boost::re_detail_500::cpp_regex_traits_base<char> const&, unsigned long) in libFlightCrew.a(Utilities.cpp.o)
      boost::cpp_regex_traits<char>::get_catalog_name() in libFlightCrew.a(Utilities.cpp.o)
      boost::re_detail_500::mem_block_cache::get() in libFlightCrew.a(Utilities.cpp.o)
      boost::re_detail_500::mem_block_cache::put(void*) in libFlightCrew.a(Utilities.cpp.o)
      boost::object_cache<boost::re_detail_500::cpp_regex_traits_base<char>, boost::re_detail_500::cpp_regex_traits_implementation<char> >::get(boost::re_detail_500::cpp_regex_traits_base<char> const&, unsigned long) in libFlightCrew.a(DetermineMimetype.cpp.o)
      boost::cpp_regex_traits<char>::get_catalog_name() in libFlightCrew.a(DetermineMimetype.cpp.o)
      boost::re_detail_500::mem_block_cache::get() in libFlightCrew.a(DetermineMimetype.cpp.o)
      ...
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

​https://build.macports.org/builders/ports-10.15_x86_64-builder/builds/187727/steps/install-port/logs/stdio

I cannot reproduce this locally with gcc.