Alex Richardson authored Feb 28, 2022 and Vlad Zahorodnii committed Mar 17, 2022

Allows removing some CMake checks config-kwin.h contents. This is
supported by all compilers and required for C++17. While touching those
lines I also cleaned up an unnecessary HAVE_UNISTD_H check (glibc always
has it and and incorrect use of HAVE_SYS_PROCCTL_H.