bootstrap: respect POSIX jobserver

When bootstrapping rust, the `-j N` flag was passed to `cmake`, which
was then forwarded to Ninja. This prevents the jobserver from being
used, and as a result leads to oversubscription when rust is just one of
the many packages built as part of a larger software stack.

Since Cargo has long supported the jobserver, it would be good if also
bootstrapping rust itself would participate in the protocol, leading to
composable parallelism.

Signed-off-by: Harmen Stoppels <me@harmenstoppels.nl>

Author: haampie

src/bootstrap/src/core/build_steps/llvm.rs

@@ -773,7 +773,12 @@ fn configure_cmake(
         .define("CMAKE_CXX_COMPILER", sanitize_cc(&cxx))
         .define("CMAKE_ASM_COMPILER", sanitize_cc(&cc));
 
-    cfg.build_arg("-j").build_arg(builder.jobs().to_string());
+    let has_jobserver =
+        env::var("MAKEFLAGS").map(|flags| flags.contains("--jobserver-auth=")).unwrap_or(false);
+
+    if !has_jobserver {
+        cfg.build_arg("-j").build_arg(builder.jobs().to_string());
+    }
     let mut cflags = ccflags.cflags.clone();
     // FIXME(madsmtm): Allow `cmake-rs` to select flags by itself by passing
     // our flags via `.cflag`/`.cxxflag` instead.

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

@@ -547,10 +547,6 @@ impl Builder<'_> {
             assert_eq!(target, compiler.host);
         }
 
-        // Remove make-related flags to ensure Cargo can correctly set things up
-        cargo.env_remove("MAKEFLAGS");
-        cargo.env_remove("MFLAGS");
-
         cargo
     }