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

third_party/harfbuzz-ng/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/linux/pkg_config.gni")
 
 # The GYP build supports system harfbuzz for non-official builds when using
 # pangoft2 1.31.0 or greater (which pulls it in).
 # TODO(brettw) we can consider doing this as well, although the benefit is
 # unclear and requires shelling out to a script to check the version.
 #
 # ChromeOS uses an up-to-date system one that we have control over, so we
 # don't want to bloat the binary more by including another copy.
 
 declare_args() {
   if (is_chromeos) {
     # ChromeOS includes an up-to-date system harfbuzz that we have control
     # over, so it use the system one by default.
     use_system_harfbuzz = true
   } else if (is_official_build) {
     # For official builds, we want to control the Harbuzz version so always
     # use our included one. Currently the sysroot includes a version of pangoft
     # that doesn't link to harfbuzz, so there are no linker problems. If we
     # 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.
     use_system_harfbuzz = exec_script(
         pkg_config_script,
         pkg_config_args + [ "--atleast-version=1.31.0", "pangoft2" ],
         "value")
+  } else {
+    use_system_harfbuzz = false
   }
 }
 
 if (use_system_harfbuzz) {
   import("//build/config/linux/pkg_config.gni")
   pkg_config("harfbuzz_pkgconfig") {
     packages = [ "harfbuzz" ]
   }
   group("harfbuzz-ng") {
     public_configs = [ ":harfbuzz_pkgconfig" ]
@@ skipping 123 matching lines @@
     }
     if (is_mac) {
       sources += [
         "src/hb-coretext.cc",
         "src/hb-coretext.h",
       ]
       defines += [ "HAVE_CORETEXT" ]
     }
   }
 }