clang: Makes builds with clang less dependent on absolute file path

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("//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.
 #
@@ skipping 23 matching lines @@
 }
 
 pkg_config_script = "//build/config/linux/pkg-config.py"
 
 # Define the args we pass to the pkg-config script for other build files that
 # need to invoke it manually.
 if (sysroot != "") {
   # Pass the sysroot if we're using one (it requires the CPU arch also).
   pkg_config_args = [
     "-s",
-    sysroot,
+    rebase_path(sysroot),
     "-a",
     current_cpu,
   ]
 } else if (pkg_config != "") {
   pkg_config_args = [
     "-p",
     pkg_config,
   ]
 } else {
   pkg_config_args = []
@@ skipping 18 matching lines @@
       ldflags = pkgresult[4]
     }
 
     forward_variables_from(invoker,
                            [
                              "defines",
                              "visibility",
                            ])
   }
 }