Test for a certain rust version at configure time
Right now we test for the presence of rust/cargo in the configure script, but we don't check the version of rustc. I have no idea what our minimum version is. It would be good to formalize this.
We should require that librsvg always builds on Rust stable, preferably a couple versions before the latest compiler, to help distros which don't update constantly. It would be especially nice to tie our rustc version to whatever the latest Firefox ESR requires.
- Figure out the oldest stable rustc version in which we compile. It's okay to stop at 1.20.0 and ignore older ones.
- Write a test in configure.ac for that rust version.