configure: Fix bashism.

/bin/sh defaults to dash on debian.

Reviewed-by: Brian Paul <brianp@vmware.com>
diff --git a/configure.ac b/configure.ac
index 258cb3d..3f75266 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1072,7 +1072,7 @@
 
     dri_drivers=`IFS=', '; echo $with_dri_drivers`
     for driver in $dri_drivers; do
-        DRI_DIRS+="$driver "
+        DRI_DIRS="$DRI_DIRS $driver"
         case "x$driver" in
         xi915)
             HAVE_I915_DRI=yes;