Use runtime env in build script (#17)

While build.rs runtime and compile-time environment coincide when using
cargo, they may not when using other build systems (notably bazel, which
is therefore unable to build a project depending on this crate).

As explained in [the docs][], build scripts should get their inputs from
the environment at runtime, not during compilation.

[the docs]:https://doc.rust-lang.org/cargo/reference/build-scripts.html#inputs-to-the-build-script

Co-authored-by: Paolo Tranquilli <redsun82@gihub.com>

Author: redsun82

build.rs

@@ -36,7 +36,7 @@ fn main() {
         });
 
     let expected_version_metadata = format!("+llvm-{}", &llvm_commit_hash[..12]);
-    let actual_version = env!("CARGO_PKG_VERSION");
+    let actual_version = std::env::var("CARGO_PKG_VERSION").expect("no CARGO_PKG_VERSION in env");
     if !actual_version.ends_with(&expected_version_metadata) {
         eprintln!("\nexpected version ending in `{expected_version_metadata}`, found `{actual_version}`\n");
         panic!("failed to validate Cargo package version (see above)");