Default DRI driver directory to match X.Org xserver

Since the only valid consumer of the DRI drivers is the X.Org xserver,
this changes the default DRI driver directory to match xorg-server:
${libdir}/dri. The old default of /usr/X11R6/modules/dri was wrong for
nearly all current systems.
diff --git a/configure.ac b/configure.ac
index e57f7b7d..8695884 100644
--- a/configure.ac
+++ b/configure.ac
@@ -443,9 +443,9 @@
 dnl Directory for DRI drivers
 AC_ARG_WITH([dri-driverdir],
     [AS_HELP_STRING([--with-dri-driverdir=DIR],
-        [directory for the DRI drivers @<:@/usr/X11R6/lib/modules/dri@:>@])],
+        [directory for the DRI drivers @<:@${libdir}/dri@:>@])],
     [DRI_DRIVER_INSTALL_DIR="$withval"],
-    [DRI_DRIVER_INSTALL_DIR='/usr/X11R6/lib/modules/dri'])
+    [DRI_DRIVER_INSTALL_DIR='${libdir}/dri'])
 AC_SUBST([DRI_DRIVER_INSTALL_DIR])
 dnl Direct rendering or just indirect rendering
 AC_ARG_ENABLE([driglx-direct],