Thanks for the bug reports.  I installed patches that should address the two 
issues you raised, though not quite in the ways you suggested.  GIFLIB before 
5.1 has problems with GifErrorString's signature; a simple workaround is to 
avoid using GifErrorString in GIFLIB 5.0.x.  Please see:

http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=6ee7eabb5dbdf39f2d7471c9e42fc90c315e6a9f

http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=f962c5d6472804f788cdd54631bcc46aab5f59ce