Fix libc++ link errors on CI on linux release builds.

https://logs.chromium.org/logs/gn/buildbucket/cr-buildbucket/8673621231167449729/+/u/release/linux-amd64/build/stdout
Change-Id: Ifab782010048073212d6e80118e649f76a6a6964
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/25340
Commit-Queue: Matt Stark <msta@google.com>
Reviewed-by: Takuto Ikuta <tikuta@google.com>

build/gen.py

diff --git a/build/gen.py b/build/gen.py
index 1f95679..031bb0c 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -449,11 +449,12 @@
     }
     ninja.CargoLibTarget(
         library_to_a('gn_starlark'),
+        cargo_flags='--features ninja',
         **starlark_common_args,
     )
     rust_tests = ninja.CargoTestTarget(
         'rust_unittests',
-        cargo_flags='--workspace',
+        cargo_flags='--workspace --features ninja',
         implicit_inputs=[
             library_to_a('base'),
             library_to_a('gn_lib'),

src/gn/starlark/Cargo.lock

diff --git a/src/gn/starlark/Cargo.lock b/src/gn/starlark/Cargo.lock
index 2a744ae..d215767 100644
--- a/src/gn/starlark/Cargo.lock
+++ b/src/gn/starlark/Cargo.lock
@@ -748,6 +748,7 @@
 version = "0.1.0"
 dependencies = [
  "ffi",
+ "link-cplusplus",
 ]
 
 [[package]]

src/gn/starlark/Cargo.toml

diff --git a/src/gn/starlark/Cargo.toml b/src/gn/starlark/Cargo.toml
index c16cdfd..6aa551e 100644
--- a/src/gn/starlark/Cargo.toml
+++ b/src/gn/starlark/Cargo.toml
@@ -12,6 +12,7 @@
 
 [dependencies]
 ffi = { path = "crates/ffi" }
+link-cplusplus = { workspace = true }
 
 [workspace]
 members = [
@@ -34,11 +35,17 @@
 # in sync unless you're trying to use new C++ features in the bridge.
 cxx = { version = "1.0", features = ["c++20"] }
 either = "1.16"
+link-cplusplus = "1.0"
 starlark = "0.14"
 starlark_derive = "0.14"
 thiserror = "2.0"
 strum = { version = "0.26", features = ["derive"] }
 
+[features]
+# When compiling with ninja, ninja has its own set of link flags.
+# Thus, we need to tell link-cplusplus not to generate any of its own.
+ninja = ["link-cplusplus/nothing"]
+
 # A short-lived fork until my PR gets merged.
 # Basically all it does is make some pub(crate) things public, so that we can
 # create records directly in rust (could only create them by running in starlark