clang: error: unknown argument: '-fvpt'

[ElDavoo]

Looks like using PGO with clang isn't an option.

[vaeth]

This is probably correct. However, 10-flag.sh already contains more than a healthy amount of implicit magic. Adding even more does not sound reasonable. You can probably use whatever mechanism you use to switch between gcc and clang to also not auto-enable PGO (e.g. unset PGO_DIR or export NOPGO=1).