GN: Only honour system_libdir for target toolchain

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 37 matching lines @@
 
 # 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,
     "-a",
     current_cpu,
-    "--system_libdir",
-    system_libdir,
   ]
 } else if (pkg_config != "") {
   pkg_config_args = [
     "-p",
     pkg_config,
-    "--system_libdir",
-    system_libdir,
   ]
 } else {
   pkg_config_args = []
 }
 
+# Only use the custom libdir when building with the default toolchain.
+if (current_toolchain == default_toolchain) {
+  pkg_config_args += [
+    "--system_libdir",
+    system_libdir,
+  ]
+}
+
 template("pkg_config") {
   assert(defined(invoker.packages),
          "Variable |packages| must be defined to be a list in pkg_config.")
   config(target_name) {
     args = pkg_config_args + invoker.packages
     if (defined(invoker.extra_args)) {
       args += invoker.extra_args
     }
 
     pkgresult = exec_script(pkg_config_script, args, "value")
     include_dirs = pkgresult[0]
     cflags = pkgresult[1]
 
     if (!defined(invoker.ignore_libs) || !invoker.ignore_libs) {
       libs = pkgresult[2]
       lib_dirs = pkgresult[3]
       ldflags = pkgresult[4]
     }
 
     forward_variables_from(invoker,
                            [
                              "defines",
                              "visibility",
                            ])
   }
 }