let us compilation choice to be honored.
https://github.com/rust-lang/rust/issues/39900

Index: src/bootstrap/lib.rs
--- src/bootstrap/lib.rs.orig
+++ src/bootstrap/lib.rs
@@ -1117,7 +1117,6 @@ impl Build {
             .args()
             .iter()
             .map(|s| s.to_string_lossy().into_owned())
-            .filter(|s| !s.starts_with("-O") && !s.starts_with("/O"))
             .collect::<Vec<String>>();
 
         // If we're compiling on macOS then we add a few unconditional flags