Check Rust version in build.rs

[sharkdp]

We receive a lot of tickets like this (#162) due to outdated versions of the Rust compiler. Presumably, there are even more people that try to build fd and abandon the project as soon as they see a compile error (without creating a ticket).

Apparently, there is a way to check the compiler version in build.rs, as suggested by @flip111. For details, see this stackoverflow answer.

[matematikaadit]

I'm interested in this.