configure.ac: Make libdrm optional in weston-launch

If libdrm is available, weston-launch and launcer-util.c will support
getting the drm device and setting and dropping drm master, otherwise
we'll only support getting input devices.
diff --git a/src/weston-launch.c b/src/weston-launch.c
index d8364c8..56e22b1 100644
--- a/src/weston-launch.c
+++ b/src/weston-launch.c
@@ -50,8 +50,6 @@
 #include <grp.h>
 #include <security/pam_appl.h>
 
-#include <xf86drm.h>
-
 #ifdef HAVE_SYSTEMD_LOGIN
 #include <systemd/sd-login.h>
 #endif
@@ -70,6 +68,26 @@
 
 #define MAX_ARGV_SIZE 256
 
+#ifdef HAVE_LIBDRM
+
+#include <xf86drm.h>
+
+#else
+
+static inline int
+drmDropMaster(int drm_fd)
+{
+	return 0;
+}
+
+static inline int
+drmSetMaster(int drm_fd)
+{
+	return 0;
+}
+
+#endif
+
 struct weston_launch {
 	struct pam_conv pc;
 	pam_handle_t *ph;