On Clang, LTO is ThinLTO, which leaves a lot of performance out for macOS

The LTO flag is passed in as ``--with-lto`

Line 430 in aa430e2

CONFIGURE_FLAGS="${CONFIGURE_FLAGS} --with-lto"

However, on version 3.12 and higher, Clang defaults to the ThinLTO policy https://docs.python.org/3.14/using/configure.html#cmdoption-with-lto

This leaves quite a bit of perf on the table for macOS on 3.13 and higher python/cpython#122580

The fix should be to pass it --with-lto=full on 3.12 and higher.

Note that this might make builds take 2x time for LTO!

Not reproducible in upstream clang. So this might be an Apple Clang bug.

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