Rust dependencies are not really making everyone happy apparently. We
don't want to make SVG support optional anymore because it is too
important, and unfortunately there doesn't seem to be any suitable
replacement SVG libs so far (or we haven't found them).

We explain this in the INSTALL file and also explain a bit how to bypass
the SVG parts for packagers really needing to do so, allowing GIMP to be
as portable as possible.

See also issue #6821.