drm: fix importing of libdrm

We need to link against libdrm to use its functions.

Signed-off-by: Enrico Weigelt, metux IT consult <enrico.weigelt@gr13.net>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

configure.ac

@@ -296,7 +296,7 @@ CAIRO_ENABLE_SURFACE_BACKEND(beos, BeOS/Zeta, no, [
 dnl ===========================================================================
 
 CAIRO_ENABLE_SURFACE_BACKEND(drm, DRM, no, [
-  drm_REQUIRES="libudev >= 136"
+  drm_REQUIRES="libudev >= 136, libdrm >= 2.4"
   PKG_CHECK_MODULES(drm, $drm_REQUIRES, ,
    [use_drm="no (requires $drm_REQUIRES, udev is available from git://git.kernel.org/pub/scm/linux/hotplug/udev.git)"])
 ])