rutabaga_gfx: fix building with llvm (#376889)

diff --git a/rutabaga_gfx/build.rs b/rutabaga_gfx/build.rs

index bd5a9be96..d2244c4c6 100644

--- a/rutabaga_gfx/build.rs

+++ b/rutabaga_gfx/build.rs

@@ -194,9 +194,14 @@ fn gfxstream() -> Result<()> {

             pkg_config::Config::new().probe("libdrm")?;

         }

+        let mut use_clang = target_os.contains("macos");

+        if std::env::var("USE_CLANG").is_ok() {

+            use_clang = true;

+        }

+

         // Need to link against libc++ or libstdc++.  Apple is clang-only, while by default other

         // Unix platforms use libstdc++.

-        if target_os.contains("macos") {

+        if use_clang {

             println!("cargo:rustc-link-lib=dylib=c++");

         } else if target_os.contains("linux") || target_os.contains("nto") {

             println!("cargo:rustc-link-lib=dylib=stdc++");

    })

    # Install the dylib on Darwin.

    ./darwin-install.patch

    # Patch for libc++, drop in next update

    # https://chromium.googlesource.com/crosvm/crosvm/+/8ae3c23b2e3899de33b973fc636909f1eb3dc98c

    ./link-cxx.patch

  ];

  env = lib.optionalAttrs stdenv.hostPlatform.useLLVM {

    USE_CLANG = true;

  };

  nativeBuildInputs = [

    cargo

    pkg-config