cmake: Some small cmake cleanups (#5572)

* remove unnecessary include

* cmake: use pkg_get_variable

We can find wayland-scanner executable and wayland-protocols dir by
taking advantage of this function, so no need to use find_program or
manually call pkgconf executable.

* cmake: remove explicit rdynamic option

CMAKE_ENABLE_EXPORTS=ON already implies rdynamic so it's redundant to
set the latter explicitly.

Also, CMAKE_ENABLE_EXPORTS is superseded by
CMAKE_EXECUTABLE_ENABLE_EXPORTS in cmake 3.27.

* cmake: make xcb-errors dep optional

xcb-errors is being used in wlroots, where it's optional. So make it
optional in hyprland as well

src/managers/eventLoop/EventLoopManager.cpp

@@ -4,7 +4,6 @@
 #include <algorithm>
 #include <limits>
 
-#include <sys/poll.h>
 #include <sys/timerfd.h>
 #include <time.h>