Don't store build check rmeta

This ensures the `.rmeta` files generated by `can_compile` are not
written to the build output. Those metadata files can create determinism
and cache invalidation issues in some build systems.

Instead the output -- since it is never actually read -- is written to
`/dev/null`.

build.rs

@@ -229,8 +229,8 @@ fn can_compile<T: AsRef<str>>(test: T) -> bool {
         .arg("--emit=metadata") // Do as little as possible but still parse.
         .arg("--target")
         .arg(target)
-        .arg("--out-dir")
-        .arg(out_dir); // Put the output somewhere inconsequential.
+        .arg("-o")
+        .arg("/dev/null"); // We don't care about the output (only whether it builds or not)
 
     // If Cargo wants to set RUSTFLAGS, use that.
     if let Ok(rustflags) = var("CARGO_ENCODED_RUSTFLAGS") {