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

build/config/posix/BUILD.gn

 # Copyright 2015 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")
 
 assert(is_posix)
 
 # This is included by reference in the //build/config/compiler:runtime_library
 # config that is applied to all targets. It is here to separate out the logic
 # that is Posix-only. Please see that target for advice on what should go in
 # :runtime_library vs. :compiler.
 config("runtime_library") {
   if (!is_mac && sysroot != "") {
     # Pass the sysroot to all C compiler variants, the assembler, and linker.
-    cflags = [ "--sysroot=" + sysroot ]
+    cflags = [ "--sysroot=" + rebase_path(sysroot, root_build_dir) ]
     asmflags = cflags
     ldflags = cflags
 
     # 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") ]
   }
 }