Fix build on macOS due to not having Wayland (variant 2)

Wayland-specific stuff is guarded by #ifdef Q_OS_UNIX, which also matches macOS.

Replaced with #ifdef HAVE_WAYLAND guard.

The guard is based on __has_include feature of C++17.

This is an alternative solution for: Bug: https://bugs.kde.org/show_bug.cgi?id=497136 MR: !334 (closed) Commit: 3d7308a7