Utilities: Activate POSIX APIs even without compiler extensions

Compile some third-party libraries with preprocessor definitions that
activate POSIX APIs even when compiler extensions are not enabled.
We already do this in libuv, inherited from the upstream buildsystem.

This extends commit f034b0f663 (CMake compilation: do not use compiler
extensions, 2020-03-14, v3.18.0-rc1~494^2).

Issue: #20454