rust-dev doesn't support building with external llvm

[semarie]

When building rustc-nightly (e2be5f5 here), the build error inside RustDev Target with:

> RustDev { target: TargetSelection { triple: "x86_64-unknown-openbsd", file: None } }
Dist RustDev (x86_64-unknown-openbsd)
thread 'main' panicked at 'Error: File "/data/semarie/build-rust/build_dir/build/x86_64-unknown-openbsd/llvm/bin/llvm-config" not found!', src/bootstrap/lib.rs:1273:17
stack backtrace:
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
        finished in 0.205

The build is done with the following config.toml:

[build]
rustc = "/data/semarie/build-rust/install_dir/beta/bin/rustc"
cargo = "/data/semarie/build-rust/install_dir/beta/bin/cargo"
rustfmt = "/data/semarie/build-rust/install_dir/beta/bin/rustfmt"
python = "/usr/local/bin/python3"
gdb = "/usr/local/bin/egdb"
#docs = false
vendor = true
extended = true
verbose = 1

[install]
prefix = "/data/semarie/build-rust/install_dir/nightly"

[dist]
src-tarball = false

[rust]
channel = "nightly"
codegen-tests = false
verbose-tests = true

[target.x86_64-unknown-openbsd]
llvm-config = "/usr/local/bin/llvm-config"

[llvm]
static-libstdcpp = false
ninja = true

Please note that llvm is system wide installed.

I am a bit unsure about the exact purpose of RustDev. but it seems to me that it should be not build if llvm is external ?

Cc: @Mark-Simulacrum

[Mark-Simulacrum]

Yes, I had hoped it would just work but I guess not. I will post a PR fixing it shortly. It is indeed not really intended for building with external LLVM.