bootstrap: Don't overwrite CFLAGS_* set in the environment

Author: madsmtm

src/bootstrap/src/core/builder/cargo.rs

@@ -317,8 +317,15 @@ impl Cargo {
             let cc = ccacheify(&builder.cc(target));
             self.command.env(format!("CC_{triple_underscored}"), &cc);
 
-            let cflags = builder.cc_unhandled_cflags(target, GitRepo::Rustc, CLang::C).join(" ");
-            self.command.env(format!("CFLAGS_{triple_underscored}"), &cflags);
+            // Extend `CXXFLAGS_$TARGET` with our extra flags.
+            let env = format!("CFLAGS_{triple_underscored}");
+            let mut cflags =
+                builder.cc_unhandled_cflags(target, GitRepo::Rustc, CLang::C).join(" ");
+            if let Ok(var) = std::env::var(&env) {
+                cflags.push(' ');
+                cflags.push_str(&var);
+            }
+            self.command.env(env, &cflags);
 
             if let Some(ar) = builder.ar(target) {
                 let ranlib = format!("{} s", ar.display());
@@ -329,11 +336,17 @@ impl Cargo {
 
             if let Ok(cxx) = builder.cxx(target) {
                 let cxx = ccacheify(&cxx);
-                let cxxflags =
+                self.command.env(format!("CXX_{triple_underscored}"), &cxx);
+
+                // Extend `CXXFLAGS_$TARGET` with our extra flags.
+                let env = format!("CXXFLAGS_{triple_underscored}");
+                let mut cxxflags =
                     builder.cc_unhandled_cflags(target, GitRepo::Rustc, CLang::Cxx).join(" ");
-                self.command
-                    .env(format!("CXX_{triple_underscored}"), &cxx)
-                    .env(format!("CXXFLAGS_{triple_underscored}"), cxxflags);
+                if let Ok(var) = std::env::var(&env) {
+                    cxxflags.push(' ');
+                    cxxflags.push_str(&var);
+                }
+                self.command.env(&env, cxxflags);
             }
         }