darwin: Define GALLIUM_DRIVERS_DIRS in darwin config

Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>
diff --git a/configs/darwin b/configs/darwin
index 9906107..9c126ea 100644
--- a/configs/darwin
+++ b/configs/darwin
@@ -57,3 +57,5 @@
 DRIVER_DIRS = osmesa
 #DRIVER_DIRS = dri
 DRI_DIRS = swrast
+GALLIUM_DRIVERS_DIRS = softpipe trace rbug noop identity galahad failover
+#GALLIUM_DRIVERS_DIRS += llvmpipe