Update giflib header to 4.2.1 and fix the giflib example

4.2.1 is the latest in the giflib 4 series.

There is a more recent giflib 5 series, but eg. Ubuntu 13.04 comes with
giflib 4, and 4 and 5 aren't ABI compatible, so it seems best to keep
the header for giflib 4 for now, instead of updating to giflib 5 directly.
Add separate header for giflib 5.0.4