Fix bug #17790 with compilation against giflib 5.1.0 and later.

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.