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

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index d89ba2cf03c3985fb25302d1c9f302b8b70f4620..00082bebe0387edbf97fb3cac68fc2417d6be27f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -736,6 +736,12 @@ group("gn_only") {
     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)",
     ]
   }