cmake fails to build on macos 10.13 (2019Q4)

[Greg Troxel <gdt%lexort.com@localhost>]

With a bootstrap more or less matching the joyent one (but no longer
using their binary packages because they are for 10.15, I think), many
things build ok, but cmake fails:

  -- The C compiler identification is AppleClang 8.0.0.8000042
  -- The CXX compiler identification is AppleClang 8.0.0.8000042
  -- Check for working C compiler: /opt/pkgsrc/devel/cmake/work/.wrapper/bin/clang
  -- Check for working C compiler: /opt/pkgsrc/devel/cmake/work/.wrapper/bin/clang -- works
  -- Detecting C compiler ABI info
  -- Detecting C compiler ABI info - done
  -- Detecting C compile features
  -- Detecting C compile features - done
  -- Check for working CXX compiler: /opt/pkgsrc/devel/cmake/work/.wrapper/bin/clang++
  -- Check for working CXX compiler: /opt/pkgsrc/devel/cmake/work/.wrapper/bin/clang++ -- works
  -- Detecting CXX compiler ABI info
  -- Detecting CXX compiler ABI info - done
  -- Detecting CXX compile features
  -- Detecting CXX compile features - done
  -- Checking if compiler supports needed C++17 constructs
  -- Checking if compiler supports needed C++17 constructs - no
  -- Checking if compiler supports needed C++14 constructs
  -- Checking if compiler supports needed C++14 constructs - no
  -- Checking if compiler supports C++ make_unique
  -- Checking if compiler supports C++ make_unique - no
  -- Checking if compiler supports C++ unique_ptr
  -- Checking if compiler supports C++ unique_ptr - no
  CMake Error at CMakeLists.txt:107 (message):
    The C++ compiler does not support C++11 (e.g.  std::unique_ptr).

This doesn't really make sense; surely clang 8 supports C++11.

Is anyone else seeing this?