gif2webp: Backward compatibility for giflib version <= 4.1.3

Fixes https://code.google.com/p/webp/issues/detail?id=172

We use 'int' instead of 'GifWord', which was introduced in version
4.1.4.

Tested OK on 4.1.3
(http://sourceforge.net/projects/giflib/files/giflib-4.x/giflib%204.1.3/)
and 5.0.4

Change-Id: I77628b0f9175a91eb3a22e64fc55dd00633add1f
1 file changed