use filterdiff tool if available to strip timestamps from example

use filterdiff tool if available to strip timestamps from example

The diff tool will emit a timestamp along with the diff. This
timestamps prevents build output from being consistent between
builds.

If the filterdiff tool is detected, use it to strip the timestamp
and allow for reproducible build output.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@1224 64e312b2-a51f-0410-8e61-82d0ca0eb02a