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.
Tasks:
-
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.