Report emcc version failure when binary is not found

Author: guusw

build.rs

@@ -235,22 +235,27 @@ fn which_freebsd() -> Option<i32> {
 }
 
 fn emcc_version_code() -> Option<u64> {
-    let output = if cfg!(target_os = "windows") {
-        std::process::Command::new("emcc.bat")
-            .arg("-dumpversion")
-            .output()
-            .ok()
+    let emcc = if cfg!(target_os = "windows") {
+        "emcc.bat"
     } else {
-        std::process::Command::new("emcc")
-            .arg("-dumpversion")
-            .output()
-            .ok()
-    }?;
+        "emcc"
+    };
 
-    if !output.status.success() {
-        return None;
+    let output = Command::new(emcc).arg("-dumpversion").output().ok();
+
+    // If we're targeting emscripten, we need to make sure the executable is checked for the right version
+    let is_targeting_emscripten =
+        env::var("CARGO_CFG_TARGET_OS").map_or(false, |os| os == "emscripten");
+    if output.is_none() {
+        if is_targeting_emscripten {
+            panic!("Failed to query emcc for version, make sure emcc is installed");
+        } else {
+            return None;
+        }
     }
 
+    let output = output.unwrap();
+
     let version = String::from_utf8(output.stdout).ok()?;
 
     // Some Emscripten versions come with `-git` attached, so split the