building lang/llvm with pkgsrc clang (was: using pkgsrc with pkgsrc clang)

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

> then re-build lang/clang.
Which fails building llvm. Which sounds crazy given it builds with gcc.

-- The C compiler identification is Clang 13.0.1
-- The CXX compiler identification is Clang 13.0.1
[...]
-- Performing Test LLVM_LIBSTDCXX_MIN
-- Performing Test LLVM_LIBSTDCXX_MIN - Success
-- Performing Test LLVM_LIBSTDCXX_SOFT_ERROR
-- Performing Test LLVM_LIBSTDCXX_SOFT_ERROR - Failed
CMake Error at cmake/modules/CheckCompilerVersion.cmake:119 (message):
  libstdc++ version should be at least 5.1 because LLVM will soon use new C++
  features which your toolchain version doesn't support.  You can temporarily
  opt out using LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN, but very soon your
  toolchain won't be supported.
Call Stack (most recent call first):
  cmake/config-ix.cmake:14 (include)
  CMakeLists.txt:684 (include)

I formerly sucessfully built lang/clang (for clang-format) on macOS (with clang, of course).
Help!