Add tests for executability of CC and CXX

Author: illicitonion

test/cargo_build_script/build.rs

@@ -4,4 +4,24 @@ fn main() {
         "cargo:rustc-env=TOOL_PATH={}",
         std::env::var("TOOL").unwrap()
     );
+
+    // Assert that the CC and CXX env vars existed and were executable.
+    // We don't assert what happens when they're executed (in particular, we don't check for a
+    // non-zero exit code), but this asserts that it's an existing file which is executable.
+    //
+    // Unfortunately we need to shlex the path, because we add a `--sysroot=...` arg to the env var.
+    for env_var in &["CC", "CXX"] {
+        let v = std::env::var(env_var)
+            .unwrap_or_else(|err| panic!("Error getting {}: {}", env_var, err));
+        let (path, args) = if let Some(index) = v.find("--sysroot") {
+            let (path, args) = v.split_at(index);
+            (path, Some(args))
+        } else {
+            (v.as_str(), None)
+        };
+        std::process::Command::new(path)
+            .args(args.into_iter())
+            .status()
+            .unwrap();
+    }
 }