Default to using gifsave for libvips >= 8.12.0

[vansante]

I use libvips without magick and I would like to start generating gif files, which is possible since libvips version 8.12.0. However govips still hardcodes the codepath to use magicksave for all gif exports. It results in the error VipsOperation: class "magicksave_buffer" not found (see #255 as well).

This PR does a version detection to check whether the libvips version >= 8.12.0 and if true, defaults to using the new gifsave function which is much faster.

This is technically backwards incompatible (you need a new dependency CGIF, the ubuntu package name is libcgif-dev) otherwise the GIF saving feature is broken. I tried doing a runtime feature detection check for cgif, but couldn't find a way to make it work. However, currently trying to save gifs without having magick also results in an error, so I'm not sure how much of a problem this is.

Let me know if I need to tweak this PR further :).

[coveralls]

Coverage increased (+0.1%) to 76.317% when pulling 14f35e5 on vansante:cgif-save into 2bff6b1 on davidbyttow:master.

[davidbyttow]

Thank you!