launcher: Wrap drmGetMagic and drmAuthMagic so we can build without libdrm

src/launcher-util.c

@@ -80,9 +80,19 @@ drm_set_master(int drm_fd)
 		return drmSetMaster(drm_fd);
 	return -EBADF;
 }
+static int
+drm_check_master(int drm_fd)
+{
+	drm_magic_t magic;
+	if (drm_fd != -1)
+		return drmGetMagic(drm_fd, &magic) != 0 ||
+		       drmAuthMagic(drm_fd, magic) != 0;
+	return 0;
+}
 #else
 static int drm_drop_master(int drm_fd) {return 0;}
 static int drm_set_master(int drm_fd) {return 0;}
+static int drm_check_master(int drm_fd) {return 1;}
 #endif
 
 int
@@ -110,10 +120,8 @@ weston_launcher_open(struct weston_launcher *launcher,
 		}
 
 		if (major(s.st_rdev) == DRM_MAJOR) {
-			drm_magic_t magic;
 			launcher->drm_fd = fd;
-			if (drmGetMagic(fd, &magic) != 0 ||
-			    drmAuthMagic(fd, magic) != 0) {
+			if (!drm_check_master(fd)) {
 				weston_log("drm fd not master\n");
 				close(fd);
 				return -1;