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.
+
+source_set("system") {
+  sources = [
+    "buffer.h",
+    "core.h",
+    "data_pipe.h",
+    "functions.h",
+    "macros.h",
+    "message_pipe.h",
+    "system_export.h",
+    "types.h",
+    "../../platform/native/system_thunks.cc",
+    "../../platform/native/system_thunks.h",
+  ]
+  defines = [ "MOJO_SYSTEM_IMPLEMENTATION" ]
+
+  if (is_android) {
+    all_dependent_configs -= [

[Chris Masone, 2014/07/14 21:38:37]

Per the Dependent settings section of
https://code.google.com/p/chromium/wiki/GNCookbook, it seems like I should be
able to just refer to this variable here, but I get the following:

ERROR at //mojo/public/c/system/BUILD.gn:21:5: Undefined variable for -=.
    all_dependent_configs -= [
    ^--------------------
I don't have something with this name in scope now.

(from
http://build.chromium.org/p/tryserver.chromium/builders/android_chromium_gn_c...)

What'd I do wrong?

[Chris Masone, 2014/07/14 21:48:22]

Hm. I think the problem is that, essentially, this target needs to dictate that
a config option be removed from all dependent targets, and that there's not
really a way to express that? I can remove the config from _this_ target, but
that's not enough. I can _add_ a config to all dependent targets, but that isn't
exactly what I want to do either...unless there's a way that a config can
specify the removal of an ldflags entry.

[Chris Masone, 2014/07/15 20:39:22]

Discussed with Brett offline, and he points out that this flag only matters with
static libraries. Since we're using a source_set here, we no longer need to
worry about this :-)

+      "//build/config/android:no_export_static_lib_symbols"
+    ]
+  }
+}