boost: fix build for non x86_64 hostPlatforms

    "link=${link}"

    "-sEXPAT_INCLUDE=${expat.dev}/include"

    "-sEXPAT_LIBPATH=${expat.out}/lib"

    (

      # The stacktrace from exception feature causes memory leaks when built

      # with libc++. For all other standard library implementations, i.e.

      # libstdc++, we must aknowledge this or stacktrace refuses to compile.

      # Issue upstream: https://github.com/boostorg/stacktrace/issues/163

      if (stdenv.cc.libcxx != null) then

        "boost.stacktrace.from_exception=off"

      else

        "define=BOOST_STACKTRACE_LIBCXX_RUNTIME_MAY_CAUSE_MEMORY_LEAK"

    )

    # TODO: make this unconditional

  ] ++ lib.optionals (stdenv.hostPlatform != stdenv.buildPlatform ||