Rename the function and move the declaration from gif_lib.h to
gif_lib_private.h to solve conflicts when some .c-file #includes
both stdlib.h and gif_lib.h.
See also https://sourceforge.net/p/giflib/bugs/110/
* gnu/packages/patches/giflib-make-reallocarray-private.patch: New
file
* gnu/local.mk (dist_patch_DATA): Register it.
* gnu/packages/image.scm(giflib)[source](patches): New field.
