trace: New pipe driver to trace incoming calls.

Only pipe_screen calls traced, and only linux supported, for now.
diff --git a/SConstruct b/SConstruct
index fe27cea..ee96bc5 100644
--- a/SConstruct
+++ b/SConstruct
@@ -46,7 +46,7 @@
 opts.Add(ListOption('statetrackers', 'state trackers to build', default_statetrackers,
                      ['mesa', 'python']))
 opts.Add(ListOption('drivers', 'pipe drivers to build', default_drivers,
-                     ['softpipe', 'failover', 'i915simple', 'i965simple', 'cell']))
+                     ['softpipe', 'failover', 'i915simple', 'i965simple', 'cell', 'trace']))
 opts.Add(ListOption('winsys', 'winsys drivers to build', default_winsys,
                      ['xlib', 'intel', 'gdi']))