Add support for sysroots to GN's pkg-config.

build/config/linux/pkg_config.gni

 # 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.
 
+import("sysroot.gni")
+
 # Defines a config specifying the result of running pkg-config for the given
 # packages. Put the package names you want to query in the "packages" variable
 # inside the template invocation.
 #
 # You can also add defines via the "defines" variable. This can be useful to
 # add this to the config to pass defines that the library expects to get by
 # users of its headers.
 #
 # Example:
 #   pkg_config("mything") {
 #     packages = [ "mything1", "mything2" ]
 #     defines = [ "ENABLE_AWESOME" ]
 #   }
 
 template("pkg_config") {
   assert(defined(packages),
         "Variable |packages| must be defined to be a list in pkg_config.")
   config(target_name) {
+    if (sysroot != "") {
+      # Pass the sysroot if we're using one (it requires the CPU arch also).
+      args = ["-s", sysroot, "-a", cpu_arch] + packages
+    } else {
+      args = packages
+    }
     pkgresult = exec_script("//build/config/linux/pkg-config.py",
-                            packages, "value")
+                            args, "value")
     include_dirs = pkgresult[0]
     cflags = pkgresult[1]
     libs = pkgresult[2]
     lib_dirs = pkgresult[3]
   }
 }