r300g: Build driver by default

Do the auto trick that is used for both i915, i965 and svga.
diff --git a/configure.ac b/configure.ac
index 4f28fc0..d01d0f1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1279,10 +1279,12 @@
     [AS_HELP_STRING([--enable-gallium-radeon],
         [build gallium radeon @<:@default=disabled@:>@])],
     [enable_gallium_radeon="$enableval"],
-    [enable_gallium_radeon=no])
+    [enable_gallium_radeon=auto])
 if test "x$enable_gallium_radeon" = xyes; then
     GALLIUM_WINSYS_DRM_DIRS="$GALLIUM_WINSYS_DRM_DIRS radeon"
     GALLIUM_DRIVERS_DIRS="$GALLIUM_DRIVERS_DIRS r300"
+elif test "x$enable_gallium_radeon" = xauto; then
+    GALLIUM_DRIVERS_DIRS="$GALLIUM_DRIVERS_DIRS r300"
 fi
 
 dnl