Make sure diff is present at configure time.

Avoid silent failures resulting in endless build loops.

BUG=aomedia:701

Change-Id: I76d3e0bdddf473fc364abf0c956fcad5f7e3103c

configure

@@ -155,6 +155,10 @@ for t in ${all_targets}; do
     [ -f "${source_path}/${t}.mk" ] && enable_feature ${t}
 done
 
+if ! diff --version >/dev/null; then
+  die "diff missing: Try installing diffutils via your package manager."
+fi
+
 if ! perl --version >/dev/null; then
     die "Perl is required to build"
 fi