Fix bug #17790 with compilation against giflib 5.1.0 and later.
src/image.c [5 < GIFLIB_MAJOR + (1 <= GIFLIB_MINOR)]: Declare the
prototype of DGifCloseFile as appropriate for older and newer
versions of giflib.
(gif_close): New function, encapsulates the differences in the
calling sequence of DGifCloseFile before v5.1.0 and after it.
(gif_load): Call gif_close instead of DGifCloseFile. Divulge the
error string where appropriate.
lisp/term/w32-win.el (dynamic-library-alist): Support giflib 5.1.0
and later.