Reuse installed LLVM during rust compilation

[dilyanpalauzov]

I download https://static.rust-lang.org/dist/rustc-1.42.0-src.tar.gz and then I want to build rustc. Since I have LLVM on my system, I want to use it, rather than compiling a separate LLVM during the rust build process.

I find no documentation describing how to reuse my installed LLVM for rust.

[jonas-schievink]

Both ./configure and config.toml document the options you need to set

[csmoe]

./configure --help, check out --llvm-root