llvm dev - Mar 2017 - 3 stage ninja bootstrap on darwin?

Is anyone using the 3 stage bootstrap described under 3-Stage
Non-Determinism at http://llvm.org/docs/AdvancedBuilds.html? I am
trying to build against the MacPorts current python2.7, ninja and make
releases, a build of current trunk for llvm, compiler-rt, cfe and
clang-tools-extra using...

% cmake -DLLVM_LINK_LLVM_DYLIB:BOOL=ON
-DCOMPILER_RT_ENABLE_IOS:BOOL=OFF -DLLVM_LIT_ARGS:STRING=-v
-DPYTHON_EXECUTABLE:FILEPATH=/opt/local/bin/python2.7
-DLLVM_ENABLE_ASSERTIONS:BOOL=OFF -DCMAKE_OSX_SYSROOT:STRING=/
-DCMAKE_OSX_DEPLOYMENT_TARGET:STRING-DLLVM_TARGETS_TO_BUILD="X86;PowerPC;ARM"
-DCMAKE_INSTALL_PREFIX:PATH=/Users/howarth/dist
-DCMAKE_BUILD_TYPE:STRING=Release -DLLVM_ENABLE_LIBCXX:BOOL=ON -G
Ninja -C /Users/howarth/3stage/cfe-5.0.0.src/cmake/caches/3-stage.cmake
../llvm-5.0.0.src
% ninja stage3

on x86_64-apple-darwin16 using the Xcode 8.3 Beta 5 devtools as the
system compiler. The build is tripping up on the error...

-- Performing Test LLVM_NO_OLD_LIBSTDCXX
-- Performing Test LLVM_NO_OLD_LIBSTDCXX - Failed
CMake Error at cmake/modules/CheckCompilerVersion.cmake:38 (message):
  Host Clang must be able to find libstdc++4.8 or newer!
Call Stack (most recent call first):
  cmake/config-ix.cmake:14 (include)
  CMakeLists.txt:559 (include)

where is seems that -DLLVM_ENABLE_LIBCXX:BOOL=ON isn't getting passed
down to from stage1 to the stage2 configuration step. Any ideas on how
to work around this for a build against the system livc+++ under
Sierra?
         Jack