Fix the Linux nightly build by adding a LLVM_CONFIG_<target-triple> dep.

[rprichard]

The RUST_TARGET_STAGE_N rule uses LLVM_LIBDIR_RUSTFLAGS_,
which expands to -L "$(llvm-config --libdir)" when the target-triple is
also a host-triple. Rather than expand to -L "" if llvm-config has not yet
been built, add a dependency on the target llvm-config.

When the target-triple is not a host-triple, the new LLVM_CONFIG_$(2)
dependency should expand to nothing.

r? alexcrichton

[rust-highfive]

r? @alexcrichton

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[rprichard]

Build #78 succeeded, despite llvm-config not existing:

/bin/sh: /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/obj/i686-unknown-linux-gnu/llvm/Release+Asserts/bin/llvm-config: No such file or directory
rustc: x86_64-unknown-linux-gnu/stage1/lib/rustlib/i686-unknown-linux-gnu/lib/libcore
/bin/sh: /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/obj/i686-unknown-linux-gnu/llvm/Release+Asserts/bin/llvm-config: No such file or directory
rustc: x86_64-unknown-linux-gnu/stage1/lib/rustlib/i686-unknown-linux-gnu/lib/liblibc
/bin/sh: /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/obj/i686-unknown-linux-gnu/llvm/Release+Asserts/bin/llvm-config: No such file or directory
rustc: x86_64-unknown-linux-gnu/stage1/lib/rustlib/i686-unknown-linux-gnu/lib/librand

With the change for #23303, -L "" became an error, so build #79 failed:

/bin/sh: /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/obj/i686-unknown-linux-gnu/llvm/Release+Asserts/bin/llvm-config: No such file or directory
rustc: x86_64-unknown-linux-gnu/stage1/lib/rustlib/i686-unknown-linux-gnu/lib/libcore
error: empty search path given via `-L`

The problem is that we're passing LLVM's library directory to all Rust crates, even libcore, which does not actually link against an LLVM library. In certain configurations, libcore is built before LLVM, so llvm-config does not exist yet.

I've only seen the problem with a non-parallel build and --host=i686-unknown-linux-gnu,x86_64-unknown-linux-gnu on a 64-bit Linux machine. I also tried a parallel build and a --target=i686-unknown-linux-gnu build, which did not reproduce the problem. I speculate that the parallel build got lucky and built the non-native LLVM quickly enough, while maybe the single-host build succeeded because LLVM was already an indirect dependency of the Rust crates.

This PR fixes the build failure by ensuring that llvm-config exists if we are going to invoke it.

[rprichard]

I hadn't intended to make LLVM_CONFIG_XXX an order-only dependency, but it is one because it's after the | character. Changing llvm-config won't inherently force rebuilding Rust crates. That's... possibly what we want? OTOH, the llvm-config --libdir could change.

I'm inclined to make it a normal dependency instead.

[alexcrichton]

Yeah I'm fine with a normal dependency as well, thanks for tracking this down!

[rprichard]

Ok, I've changed LLVM_CONFIG_XXX into a normal dependency.

[alexcrichton]

@bors: r+ b07a1df p=1

[bors]

⌛ Testing commit b07a1df with merge 883551b...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt