Improve error message with llvm-config cannot be found.

[MurraySobol]

When I ran this command: cargo install --git https://github.com/immunant/c2rust.git c2rust
I got this error:
Compiling pest_meta v2.7.13
error: failed to run custom build command for c2rust-ast-exporter v0.19.0 (C:\Users\murra\.cargo\git\checkouts\c2rust-2a3ef4c32beab81a\516b30e\c2rust-ast-exporter)

Caused by:
process didn't exit successfully: C:\Users\murra\AppData\Local\Temp\cargo-installBzMHID\release\build\c2rust-ast-exporter-984e14db3215a3eb\build-script-build (exit code: 101)
--- stderr
thread 'main' panicked at c2rust-build-paths\src\lib.rs:88:44:
called Result::unwrap() on an Err value: Os { code: 2, kind: NotFound, message: "The system cannot find the file specified." }
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...
error: failed to compile c2rust v0.19.0 (https://github.com/immunant/c2rust.git#516b30e3), intermediate artifacts can be found at C:\Users\murra\AppData\Local\Temp\cargo-installBzMHID.
To reuse those artifacts with a future compilation, set the environment variable CARGO_TARGET_DIR to that path.

It appears, to me, that a file is missing??
Please advise.
Thanks
Murray

[thedataking]

The error message could be better but the error is likely a failure to pick up llvm-config. You can set LLVM_CONFIG_PATH or LLVM_LIB_DIR to avoid this problem assuming your system has the prerequisites installed.