🐳 Move linux pkg_config() calls into separate BUILD.gn files

BUILD.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.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 718 matching lines @@
   }
 
   if (enable_nacl) {
     deps += [ "//native_client_sdk/src:nacl_core_sdk" ]
   }
 
   if (is_android) {
     deps += [
       "//build/android/gyp/test:hello_world",
       "//build/android/incremental_install:bootstrap_java",
+
+      # Pull these in early as an optimization (makes gn gen faster).
+      "//build/config/linux(//build/toolchain/linux:clang_x64)",

[brettw, 2016/04/22 19:30:21]

We should be more careful about bringing things in. Here you're bringing in x86
versions of these files. Unless you're targeting x86, these will otherwise never
be needed (the local system will be x64) and you'll actually run pkg_config an
extra and unnecessary time.

Use "...($host_toolchain)" for both targets here, and that should be sufficient.

Can you also add to the comment (I realize why this case is particularly bad
now):

"These run expensive scripts in the host toolchain. Generally host toolchain
targets are loaded in the later part of the run, and the result is they push out
the end of generation. By preloading these, the scripts can be parallelized with
the rest of the load."

[agrieve, 2016/04/25 14:45:35]

Added a comment. Left in x86 and added a subsequent comment. V8 uses it for
snapshot generation.

+      "//build/config/linux(//build/toolchain/linux:clang_x86)",
+      "//build/config/posix(//build/toolchain/linux:clang_x64)",
+      "//build/config/posix(//build/toolchain/linux:clang_x86)",
     ]
   }
 
   if (is_linux && use_ozone) {
     deps += [
       "//headless",
       "//headless:headless_tests",
     ]
   }
 
@@ skipping 161 matching lines @@
   assert(target_name != "")  # Mark as used.
   sources = invoker.actual_sources
   assert(
       sources == invoker.actual_sources,
       "Do not use a platform name in your output directory (found \"$root_build_dir\"). http://crbug.com/548283")
 }
 
 assert_valid_out_dir("_unused") {
   actual_sources = [ "$root_build_dir/foo" ]
 }