Configuration needs to come from <execution>, <algorithm>, etc.

[rodgert]

Summary

Directly including pstl_config.h in implementation files is
incompatible with inclusion into a standard library implementation,
which will have it's own config.h.

[rodgert]

Committed by other, non-phab, means

include/pstl/internal/numeric_impl.h

 // -*- C++ -*-
 //===-- numeric_impl.h ----------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef __PSTL_numeric_impl_H
 #define __PSTL_numeric_impl_H
 
 #include <iterator>
 #include <type_traits>
 #include <numeric>
 
-#include "pstl_config.h"
 #include "execution_impl.h"
 #include "unseq_backend_simd.h"
 
 #if __PSTL_USE_PAR_POLICIES
 #include "parallel_backend.h"
 #endif
 
 namespace __pstl

include/pstl/internal/unseq_backend_simd.h

 // -*- C++ -*-
 //===-- unseq_backend_simd.h ----------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef __PSTL_unseq_backend_simd_H
 #define __PSTL_unseq_backend_simd_H
 
 #include <type_traits>
 
-#include "pstl_config.h"
 #include "utils.h"
 
 // This header defines the minimum set of vector routines required
 // to support parallel STL.
 namespace __pstl
 {
 namespace unseq_backend
 {