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, "", ".."),

[brettw, 2016/01/06 18:15:07]

Wait, I don't understand this line. Why the ".."?

[Zachary Forman, 2016/01/06 23:20:49]

Without the .., you get output like
"-I../../build/build/linux/debian_wheezy_amd64-sysroot/usr/include/glib-2.0" -
note the "build/build".

[brettw, 2016/01/07 19:22:22]

Semantically I don't think this line makes sense, and if it's working for you I
suspect it's a coincidence.

Sysroot is an absolute path. This line says "take the absolute path and make it
relative to the current directory ("build/config/linux") assuming that the
source is in the the previous directory ("build/config")."

What directory are you specifically trying to pass to the -s command, and where
should it be expressed relative to?

[Zachary Forman, 2016/01/07 22:54:27]

I don't think this is the case. To start with,the point of this change is that
we're making sysroots relative paths, and this change converts a relative path
sysroot from the output directory to the correct absolute path.

print("sysroot        = $sysroot")
print("rebased        = " + rebase_path(sysroot, "", ".."))
print("rebased w/o .. = " + rebase_path(sysroot, "", ".."))
->
sysroot        = ../../build/linux/debian_wheezy_amd64-sysroot
rebased        =
/usr/local/google/home/zforman/chromium-1/src/build/linux/debian_wheezy_amd64-sysroot
rebased w/o .. =
/usr/local/google/home/zforman/chromium-1/src/build/build/linux/debian_wheezy_amd64-sysroot

What this is doing is then taking ../../build/... and rebasing it to an absolute
path from.

This file is located in build/config/linux/ - therefore we absolutify
(build/config/linux/../)(../../build/linux/debian_wheezy_amd64-sysroot) ->
build/linux/debian_wheezy_amd64-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",
                            ])
   }
 }