Include copy of GifQuantizeBuffer function from giflib

From https://github.com/coin3d/simage/pull/33
giflib 5.1.9 and above removes the GifQuantizeBuffer() function used in
simage_gif.c (see https://sourceforge.net/p/giflib/bugs/132/ and
https://sourceforge.net/p/giflib/bugs/142/).

The fix is either to remove the related check in simage_gif.c or to include a
copy of the function as encouraged by giflib, due the util library not being
included in installed versions of giflib (license is MIT):

    The undocumented and deprecated GifQuantizeBuffer() entry point has been
    moved to the util library to reduce libgif size and attack surface.
    Applications needing this function are couraged to link the util library or
    make their own copy.