MS-Windows follow-up to 2013-10-10T01:03:11Z!rgm@gnu.org: support giflib 5.x.

 src/image.c (GIFLIB_MAJOR): Define to 4 if undefined.
 (GIFLIB_MINOR, GIFLIB_RELEASE): Define to zero if undefined.
 (GifErrorString) [GIFLIB_MAJOR >= 5]: Define a function pointer.
 (gif_load): For giflib v5.x and later, display the error message
 produced by giflib when its functions fail.
 (syms_of_image) <Qlibgif_version> [HAVE_NTGUI]: New DEFSYM.

 lisp/term/w32-win.el (dynamic-library-alist): Define separate lists
 of GIF DLLs for versions before and after 5.0.0 of giflib.

Fixes: debbugs:15531