GN: Add an option to build against the Debian wheezy sysroot

build/config/linux/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.
 
 import("//build/config/sysroot.gni")
 
 config("sdk") {
   if (sysroot != "") {
     cflags = [ "--sysroot=" + sysroot ]
     ldflags = [ "--sysroot=" + sysroot ]
 
     # Need to get some linker flags out of the sysroot.
     ldflags += [ exec_script("sysroot_ld_path.py",
                              [
                                rebase_path("//build/linux/sysroot_ld_path.sh",
                                            root_build_dir),
                                sysroot,
                              ],
                              "value") ]
+
+    # When using the pulled wheezy sysroot with gcc, we have to specify these
+    # excplicitly.
+    if (dart_use_wheezy_sysroot && !is_clang) {
+      cflags += [
+        "-I=/usr/include/c++/4.6",
+        "-I=/usr/include/c++/4.6/i486-linux-gnu",
+      ]
+    }
   }
 }