GN: Get Mac build working for IPC target at least.

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")
+import("//build/config/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:
@@ skipping 13 matching lines @@
       args = packages
     }
     pkgresult = exec_script("//build/config/linux/pkg-config.py",
                             args, "value")
     include_dirs = pkgresult[0]
     cflags = pkgresult[1]
     libs = pkgresult[2]
     lib_dirs = pkgresult[3]
   }
 }