Add Linux SDK to GN build.

build/config/BUILDCONFIG.gn

 # Copyright (c) 2013 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.
 
 # =============================================================================
 # BUILD FLAGS
 # =============================================================================
 #
 # This block lists input arguments to the build, along with their default
 # values. GN requires listing them explicitly so it can validate input and have
@@ skipping 318 matching lines @@
   "//build/config:my_msvs",  # TODO(brettw) eraseme
 
   "//build/config/compiler:compiler",
   "//build/config/compiler:chromium_code",
   "//build/config/compiler:default_warnings",
   "//build/config/compiler:no_rtti",
   "//build/config/compiler:runtime_library",
 ]
 if (is_win) {
   native_compiler_configs += [ "//build/config/win:sdk", ]
+} else if (is_linux) {
+  native_compiler_configs += [ "//build/config/linux:sdk", ]
 } else if (is_mac) {
   native_compiler_configs += [ "//build/config/mac:sdk", ]
 } else if (is_ios) {
   native_compiler_configs += [ "//build/config/ios:sdk", ]
 }
 if (is_clang) {
   native_compiler_configs += [ "//build/config/clang:find_bad_constructs" ]
 }
 
 # Optimizations and debug checking.
@@ skipping 97 matching lines @@
 } else if (is_android) {
   host_toolchain = "//build/toolchain/linux:$build_cpu_arch"
   set_default_toolchain("//build/toolchain/android:$cpu_arch")
 } else if (is_linux) {
   host_toolchain = "//build/toolchain/linux:$build_cpu_arch"
   set_default_toolchain("//build/toolchain/linux:$cpu_arch")
 } else if (is_mac || is_ios) {
   host_toolchain = "//build/toolchain/mac:clang"
   set_default_toolchain(host_toolchain)
 }