Add fixme

Author: bjorn3

build_system/main.rs

@@ -185,6 +185,7 @@ fn main() {
     env::set_var("RUSTDOC", "rustdoc_should_be_set_explicitly");
 
     let cg_clif_dylib = if use_existing_backend {
+        // FIXME also allow using llvm for comparison purposes
         CodegenBackend::Builtin("cranelift".to_owned())
     } else {
         CodegenBackend::Local(build_backend::build_backend(