Right now, we are adding systemd and elogind backends to the build
system as soon as their libraries are found on the build system.
Instead, we should only add them if the libraries have been found _and_
the user has actually requested them to be included.
Without this patch I'm getting the following build error when building
with Nix:
FAILED: backend/wlr_backend@sta/session_direct-ipc.c.o
In file included from ../backend/session/direct-ipc.c:20:0:
/nix/store/9ac27wk5vh47p28gladbdfafpidrx9rh-libdrm-2.4.88-dev/include/xf86drm.h:40:17: fatal error: drm.h: No such file or directory
#include <drm.h>
^
compilation terminated.
* Moved os-compatibility.c to util
* Added header under util
* Removed static since it isn't needed (i think so)
* Adjusted meson.build to include lib_wlr
Improved some codestyle
* Added guard to os-compatibility.h
* Fixed typo in include statment
Adjusted Guard
* Changed guard to _WLR_UTIL_OS_COMPATIBILITY
