cmake: Fix `-pthread` flags in summary

This PR removes the raw CMake generator expressions from the summary output.

Fixes https://github.com/bitcoin/bitcoin/issues/31482.

Here are a few examples of the summary:

• on FreeBSD: https://github.com/hebasto/bitcoin-core-nightly/actions/runs/12930644678/job/36062648395
• on OpenBSD: https://github.com/hebasto/bitcoin-core-nightly/actions/runs/12930647880/job/36062659528
• on NetBSD: https://github.com/hebasto/bitcoin-core-nightly/actions/runs/12930651817/job/36062672223
• on OmniOS: https://github.com/hebasto/bitcoin-core-nightly/actions/runs/12930654711/job/36062682690