Make sure diff is present at configure time.

This avoids an endless build loop at vpx_version.h
creation time when diff is not present.

Change-Id: I16ae386dbdaf14f9a2b85e4c5d1aaa6c08f52a45

configure

@@ -170,11 +170,14 @@ 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
 
-
 if [ "`cd \"${source_path}\" && pwd`" != "`pwd`" ]; then
   # test to see if source_path already configured
   if [ -f "${source_path}/vpx_config.h" ]; then