How to build Clang to use libc++ and PSTL?

Question

The code below will build and run on my Ubuntu 23.10 system using libstdc++, either with GCC or clang (via g++ execution.cpp -ltbb or clang++ execution.cpp -ltbb):

#include <vector>
#include <execution>
#include <cassert>

int main(int argc, char *argv[])
{
  using namespace std;

  vector v{1,2,3,4,5,6,7,8};
  for_each(std::execution::seq,       v.begin(), v.end(), [](auto& x) { x++; });
  for_each(std::execution::par,       v.begin(), v.end(), [](auto& x) { x++; });
  for_each(std::execution::par_unseq, v.begin(), v.end(), [](auto& x) { x++; });
  for_each(std::execution::seq,       v.begin(), v.end(), [](auto& x) { x++; });
  assert((v==vector{5,6,7,8,9,10,11,12}));
  return 0;
}

However, if I try to use Clang with libc++ (via clang++ -stdlib=libc++ execution.cpp -ltbb), compilation fails. The execution namespace isn't found, and the first error is:

error: no member named 'execution' in namespace 'std'; did you mean 'exception'?

I have read that a solution is to build LLVM/Clang with PSTL enabled, and so I configured CMake using:

cmake ../llvm \
-DLLVM_USE_LINKER=gold \
-DCMAKE_INSTALL_PREFIX=$PWD/../install_pstl \
-DCMAKE_BUILD_TYPE=Release \
-DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind;compiler-rt" \
-DLLVM_ENABLE_PROJECTS="clang;pstl" \
-DPSTL_PARALLEL_BACKEND=tbb

I then try to compile the earlier program using:

clang++ -std=c++23 -stdlib=libc++ -Wl,-rpath,"$HOME/llvm-project-main/install_pstl/lib:$HOME/llvm-project-main/install_pstl/lib/x86_64-unknown-linux-gnu:$LD_LIBRARY_PATH" execution.cpp

The program again fails to compile. Now the execution namespace is found, but none of the 4 constants (seq,par,unseq and par_unseq). The first error message is:

error: no member named 'seq' in namespace 'std::execution'

Is my configuration wrong? Is there anything I should change in the compiler invocation?

There has been prior discussion of an LLVM bug here, but that has since been closed.