Make *some* version of the Mac GN build work.

third_party/harfbuzz-ng/BUILD.gn

Index: third_party/harfbuzz-ng/BUILD.gn
diff --git a/third_party/harfbuzz-ng/BUILD.gn b/third_party/harfbuzz-ng/BUILD.gn
index d48eb08d572c632a0ee433a0c1090bd12bd088cb..195c5c81c806c752b196ecbe685fc2381a485e93 100644
--- a/third_party/harfbuzz-ng/BUILD.gn
+++ b/third_party/harfbuzz-ng/BUILD.gn
@@ -24,7 +24,7 @@ declare_args() {
     # update that version, we'll need to work around the duplicate symbols some
     # other way.
     use_system_harfbuzz = false
-  } else {
+  } else if (is_linux) {
     # Use the system harfbuzz for newer versions of pangoft, and not for older
     # ones. pangoft links to the system harfbuzz starting with 1.31.0, which
     # causes duplicate symbols when we link our own version.
@@ -32,6 +32,8 @@ declare_args() {
         pkg_config_script,
         pkg_config_args + [ "--atleast-version=1.31.0", "pangoft2" ],
         "value")
+  } else {
+    use_system_harfbuzz = false
   }
 }