With OpenBSD 7.3, the waitid(2) system call is implemented (see https://github.com/openbsd/src/commit/8112871f19bbd25e86c93d0f901071ca2335a352, first mentioned kernel improvement at https://www.openbsd.org/73.html).
This means Boost.Process finally doesn't fail to compile anymore and we can remove the build hint about missing external signer support. Tested on my amd64 machine by reconfiguring / rebuilding master branch and successfully running the functional test wallet_signer.py. :heavy_check_mark: