Thanks to Xiang Zhang for the patch.

required for PGO linking to be a configure time error rather than
make time when --with-optimizations is enabled.  Also improve our
ability to find the llvm-profdata tool on MacOS and some Linuxes.

required for PGO linking - to be a configure time error rather than
make time when --with-optimizations is enabled.  Also improve our
ability to find the llvm-profdata tool on MacOS and some Linuxes.

Original patch by Martin Panter, new policy fixes by me.

Patch by Berker Peksag.

This completes PEP 523.