CMake config scripts emulate autoconf behaviour by performing discovery of available API calls (POSIX, Linux, etc..).
This behaviour is obsolete for the platforms we support. All of them provide required API's for many years already.
Selected extensions – e.g. Linux POSIX extensions, Apple-specific functions – are easily discoverable by checking definitions provided by compilers and system libraries.

Solution:

• Remove obsolete autoconf-like discovery of well-known API.
• Use compiler definitions to detect availability.