GN: nacl_defines: Drop _BSD_SOURCE, _DEFAULT_SOURCE; conditionalize on OS

build/config/nacl/BUILD.gn

 # Copyright (c) 2014 The Native Client 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/nacl/config.gni")
 
 # Native Client Definitions
 config("nacl_defines") {
   defines = [
-    "_DEFAULT_SOURCE=1",
-    "_BSD_SOURCE=1",
     "_POSIX_C_SOURCE=199506",
     "_XOPEN_SOURCE=600",
     "_GNU_SOURCE=1",
+    "_DARWIN_C_SOURCE=1",

[Mark Seaborn, 2015/10/20 00:21:00]

It seems like some of the reasoning from the commit message should go in
comments here.

In the commit message, can you point to
https://codereview.chromium.org/1411473004/ for context, and mention that you're
syncing the GN build with that?

     "__STDC_LIMIT_MACROS=1",
   ]
 }
 
 config("nexe_defines") {
   defines = [
     "DYNAMIC_ANNOTATIONS_ENABLED=1",
     "DYNAMIC_ANNOTATIONS_PREFIX=NACL_",
   ]
 }
@@ skipping 61 matching lines @@
     "-Os",
 
     # These are omitted from non-IRT libraries to keep the libraries
     # themselves small.
     "-ffunction-sections",
     "-fdata-sections",
   ]
 
   ldflags = [ "-Wl,--gc-sections" ]
 }