Check for rust-src in build.rs

[parasyte]

cargo-xbuild is a dependency for building the MIPS sysroot, and core, alloc, and compiler-builtins crates. rust-src is a dependency of cargo-xbuild.

These dependencies should be checked in build.rs and error out with a clear list of what's missing and how to install it.

[Lokathor]

I wouldn't appreciate a build.rs file installing programs without a specific off-by-default feature being enabled for that build.

Instead, consider having the build.rs check for any dependencies and error out with a clear list of what's missing and how to install it.