Add GN build targets for the rest of mojo/public

mojo/public/c/system/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.
+
+config("exclude_all_libs") {
+  ldflags = [ "-Wl,--exclude-libs=ALL" ]

[Chris Masone, 2014/07/14 19:43:42]

I _think_ this is the right way to translate the clause where this argument is
filtered out of ldflags in the GYP mojo_system target, but I'm not sure.

+}
+
+source_set("system") {
+  sources = [
+    "buffer.h",
+    "core.h",
+    "data_pipe.h",
+    "functions.h",
+    "macros.h",
+    "message_pipe.h",
+    "system_export.h",
+    "types.h",
+    "../../../public/platform/native/system_thunks.cc",
+    "../../../public/platform/native/system_thunks.h",

[Chris Masone, 2014/07/14 19:43:41]

I'm also not sure this is the right way to reference these files, which are
outside of the directory where this file lives. Should I instead create a
BUILD.gn in that directory with a source_set including just these files, and
then reference that here?

[yzshen1, 2014/07/14 19:53:11]

I think it should be fine because I have seen this pattern elsewhere.
On 2014/07/14 19:43:41, Chris Masone wrote:

[Chris Masone, 2014/07/14 19:58:54]

Acknowledged.

+  ]
+  defines = [ "MOJO_SYSTEM_IMPLEMENTATION" ]
+
+  if (is_android) {
+    all_dependent_configs -= [ ":no_exclude_all_libs" ]

[yzshen1, 2014/07/14 19:53:11]

Mismatch name?
(I assume you want to use exclude_all_libs?)

IIUC, this won't work because "exclude_all_libs" is not a config that already
exists in the list of configs.

[Chris Masone, 2014/07/14 19:58:54]

That's what I was concerned about. So I should go to the place where this flag
is originally added to ldflags and instead break it out into a separate config?
And then make sure it gets pulled in by default under whatever conditions this
flag is currently being set, right?

+  }
+}