Add comment to explain why CG_RUSTFLAGS is needed

build_system/src/build.rs

@@ -150,6 +150,8 @@ pub fn build_sysroot(env: &HashMap<String, String>, config: &ConfigInfo) -> Resu
         "debug"
     };
 
+    // We have a different environment variable than RUSTFLAGS to make sure those flags are only
+    // sent to rustc_codegen_gcc and not the LLVM backend.
     if let Ok(cg_rustflags) = std::env::var("CG_RUSTFLAGS") {
         rustflags.push(' ');
         rustflags.push_str(&cg_rustflags);

build_system/src/config.rs

@@ -381,6 +381,8 @@ impl ConfigInfo {
         }
 
         // This environment variable is useful in case we want to change options of rustc commands.
+        // We have a different environment variable than RUSTFLAGS to make sure those flags are
+        // only sent to rustc_codegen_gcc and not the LLVM backend.
         if let Some(cg_rustflags) = env.get("CG_RUSTFLAGS") {
             rustflags.extend_from_slice(&split_args(&cg_rustflags)?);
         }