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.

configs/default

@@ -88,7 +88,7 @@ GLCORE_LIB_DEPS = -lm -lpthread
 
 # Installation directories (for make install)
 INSTALL_DIR = /usr/local
-DRI_DRIVER_INSTALL_DIR = /usr/X11R6/lib/modules/dri
+DRI_DRIVER_INSTALL_DIR = $(INSTALL_DIR)/$(LIB_DIR)/dri
 
 # Where libGL will look for DRI hardware drivers
 DRI_DRIVER_SEARCH_DIR = $(DRI_DRIVER_INSTALL_DIR)

configure.ac

@@ -443,9 +443,9 @@ AC_ARG_ENABLE([glx-tls],
 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],

docs/autoconf.html

@@ -178,8 +178,7 @@ drivers.
 <ul>
 <li><code>--with-dri-driverdir=DIR</code> - This option specifies the
 location the DRI drivers will be installed to and the location libGL
-will search for DRI drivers. The default is
-<code>/usr/X11R6/lib/modules/dri</code>.
+will search for DRI drivers. The default is <code>${libdir}/dri</code>.
 </li>
 <li><code>--with-dri-drivers=DRIVER,DRIVER,...</code> - This option
 allows a specific set of DRI drivers to be built. For example,

src/glx/x11/dri_glx.c

@@ -84,7 +84,7 @@ struct __GLXDRIconfigPrivateRec {
 
 #ifndef DEFAULT_DRIVER_DIR
 /* this is normally defined in Mesa/configs/default with DRI_DRIVER_SEARCH_PATH */
-#define DEFAULT_DRIVER_DIR "/usr/X11R6/lib/modules/dri"
+#define DEFAULT_DRIVER_DIR "/usr/local/lib/dri"
 #endif
 
 static void InfoMessageF(const char *f, ...)