Librsvg needs explicit configuration for debian testing

After this debian issue and this commit gdk-pixbuf-loader is no longer in the PATH in Debian testing and needs to be explicitly passed during configuration.

Thus till this issue is resolved I am gonna disable debian testing builds from the CI. We got to be careful onward though since our rustc version tests also use debian testing. (Maybe I could use the workaround just for the CI for a while?)

This affects Debian testing, probably Ubuntu 18:10 and our COMPILING.md docs. Ideally with the solution the user would to have to workaround this.

I got no idea if its possible to append the configure argument only for Debian derivatives with autotools.