Got it to build

CMake/FindSystemd.cmake

@@ -0,0 +1,40 @@
+#.rst:
+# FindSystemd
+# -------
+#
+# Find Systemd library
+#
+# Try to find Systemd library on UNIX systems. The following values are defined
+#
+# ::
+#
+#   SYSTEMD_FOUND         - True if Systemd is available
+#   SYSTEMD_INCLUDE_DIRS  - Include directories for Systemd
+#   SYSTEMD_LIBRARIES     - List of libraries for Systemd
+#   SYSTEMD_DEFINITIONS   - List of definitions for Systemd
+#
+#=============================================================================
+# Copyright (c) 2015 Jari Vetoniemi
+#
+# Distributed under the OSI-approved BSD License (the "License");
+#
+# This software is distributed WITHOUT ANY WARRANTY; without even the
+# implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+# See the License for more information.
+#=============================================================================
+
+include(FeatureSummary)
+set_package_properties(Systemd PROPERTIES
+   URL "http://freedesktop.org/wiki/Software/systemd/"
+   DESCRIPTION "System and Service Manager")
+
+find_package(PkgConfig)
+pkg_check_modules(PC_SYSTEMD QUIET libsystemd)
+find_library(SYSTEMD_LIBRARIES NAMES systemd ${PC_SYSTEMD_LIBRARY_DIRS})
+find_path(SYSTEMD_INCLUDE_DIRS systemd/sd-login.h HINTS ${PC_SYSTEMD_INCLUDE_DIRS})
+
+set(SYSTEMD_DEFINITIONS ${PC_SYSTEMD_CFLAGS_OTHER})
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(SYSTEMD DEFAULT_MSG SYSTEMD_INCLUDE_DIRS SYSTEMD_LIBRARIES)
+mark_as_advanced(SYSTEMD_INCLUDE_DIRS SYSTEMD_LIBRARIES SYSTEMD_DEFINITIONS)

CMakeLists.txt

@@ -51,6 +51,7 @@ find_package(DRM REQUIRED)
 find_package(LibInput REQUIRED)
 find_package(Udev)
 find_package(Dbus)
+find_package(Systemd)
 
 include(Wayland)
 include(Manpage)

backend/CMakeLists.txt

@@ -1,6 +1,7 @@
 include_directories(
 	${PROTOCOLS_INCLUDE_DIRS}
 	${WAYLAND_INCLUDE_DIR}
+	${DRM_INCLUDE_DIRS}
 )
 
 add_library(wlr-backend
@@ -8,6 +9,11 @@ add_library(wlr-backend
     wayland/registry.c
     wayland/wl_seat.c
     wayland/wl_output.c
+    drm/drm.c
+    drm/event.c
+    drm/otd.c
+    drm/session.c
+    drm/udev.c
 )
 
 target_link_libraries(wlr-backend

backend/drm/drm.c

@@ -1,6 +1,6 @@
-#include "otd.h"
-#include "drm.h"
-#include "event.h"
+#include "backend/drm/otd.h"
+#include "backend/drm/drm.h"
+#include "backend/drm/event.h"
 
 #include <assert.h>
 #include <stdio.h>

backend/drm/event.c

@@ -1,7 +1,7 @@
-#include "otd.h"
-#include "event.h"
-#include "drm.h"
-#include "udev.h"
+#include "backend/drm/otd.h"
+#include "backend/drm/event.h"
+#include "backend/drm/drm.h"
+#include "backend/drm/udev.h"
 
 #include <stdbool.h>
 #include <stdlib.h>

backend/drm/otd.c

@@ -2,11 +2,11 @@
 #include <stdlib.h>
 #include <stdio.h>
 
-#include "otd.h"
-#include "drm.h"
-#include "event.h"
-#include "session.h"
-#include "udev.h"
+#include "backend/drm/otd.h"
+#include "backend/drm/drm.h"
+#include "backend/drm/event.h"
+#include "backend/drm/session.h"
+#include "backend/drm/udev.h"
 
 struct otd *otd_start(void)
 {

backend/drm/session.c

@@ -1,3 +1,5 @@
+#define _POSIX_C_SOURCE 200809L
+
 #include <stdio.h>
 #include <stdbool.h>
 #include <stdlib.h>
@@ -9,8 +11,8 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 
-#include "session.h"
-#include "otd.h"
+#include "backend/drm/session.h"
+#include "backend/drm/otd.h"
 
 int take_device(struct otd *restrict otd,
 		       const char *restrict path,

backend/drm/udev.c

@@ -1,3 +1,5 @@
+#define _POSIX_C_SOURCE 200809L
+
 #include <libudev.h>
 #include <stdbool.h>
 #include <stdlib.h>
@@ -6,10 +8,10 @@
 #include <xf86drm.h>
 #include <xf86drmMode.h>
 
-#include "otd.h"
-#include "udev.h"
-#include "session.h"
-#include "drm.h"
+#include "backend/drm/otd.h"
+#include "backend/drm/udev.h"
+#include "backend/drm/session.h"
+#include "backend/drm/drm.h"
 
 static bool device_is_kms(struct otd *otd, struct udev_device *dev)
 {