Make sure diff is present at configure time.
Make sure diff is present at configure time.

Avoid silent failures resulting in endless build loops.

BUG=aomedia:701

Change-Id: I76d3e0bdddf473fc364abf0c956fcad5f7e3103c

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom