GIF: add compatibility with giflib 5.1 (#5519)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27456 f0d54148-0727-0410-94bb-9a71ac55c965
Add test for #5451

git-svn-id: https://svn.osgeo.org/gdal/trunk@27457 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx