gd: 2.0.35 -> 2.1.1

Upstream claims 2.1 is fully API compatible with 2.0

https://libgd.github.io/release-2.1.0.html
https://libgd.github.io/release-2.1.1.html

Also includes meta updates, adds pkg-config to the build environment,
for proper detection of dependencies, and adds optional support for tiff
and xpm image formats.