Determine the lowest rustc version we will compile with
In our docs, we say that we currently develop with nightly, and that we might stop doing that soon. In our Makefiles, we refuse to compile if rustc is <1.14. About a week ago, some of us were pretty excited that the current stable compiler (1.24) will now abort if a panic occurs at an FFI boundary. Also, our Makefiles currently contain a linker workaround for OSX systems for rustc <1.24 (legacy/trac#25341 (moved)).
We should maaaaaybe narrow down which version(s) of Rust we're using?