gcapi_mac: Add gn build file.

chrome/installer/gcapi_mac/BUILD.gn

+# Copyright 2017 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/sysroot.gni")
+
+assert(is_mac)
+
+static_library("gcapi_lib") {
+  sources = [
+    "gcapi.h",
+    "gcapi.mm",
+  ]
+
+  libs = [ "Cocoa.framework" ]
+
+  # Don't use runtime_library, to be able to pick a custom mmacosx-version-min.
+  configs -= [ "//build/config/compiler:runtime_library" ]
+  cflags = [
+    "-isysroot",
+    sysroot,
+    "-mmacosx-version-min=10.8",

[Mark Mentovai, 2017/04/25 20:08:55]

Was this at 10.8 when we deleted the GYP files?

[Nico, 2017/04/25 20:13:49]

I don't know, I just made up some number.

https://codereview.chromium.org/2321703002/diff/40001/chrome/chrome_installer...
-> looks like this used to be 10.5. (It also used to pass -fobj-gc, which clang
doesn't even understand anymore.)

+  ]
+  ldflags = [
+    "-isysroot",
+    sysroot,
+    "-mmacosx-version-min=10.8",
+  ]
+
+  complete_static_lib = true
+}
+
+executable("gcapi_example") {
+  sources = [
+    "gcapi_example_client.mm",

[Mark Mentovai, 2017/04/25 20:08:55]

You should pick up the cflags and ldflags (and I guess not runtime_library) used
by gcapi_lib in this target too.

[Nico, 2017/04/25 20:13:49]

In the gyp files, we didn't do that, but sure, done.

+  ]
+
+  deps = [
+    ":gcapi_lib",
+  ]
+}