libc++: Don't pass -pthread to link.

third_party/libc++/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/sanitizers/sanitizers.gni")
 import("//build/toolchain/toolchain.gni")
 
 # Used by libc++ and libc++abi.
 config("config") {
   defines = [ "LIBCXX_BUILDING_LIBCXXABI" ]
@@ skipping 49 matching lines @@
   configs += [
     ":config",
     "//build/config/compiler:no_chromium_code",
     "//build/config/compiler:rtti",
     "//build/config/gcc:symbol_visibility_default",
     "//build/config/sanitizers:sanitizer_options_link_helper",
   ]
 
   ldflags = [ "-nodefaultlibs" ]
 
-  # TODO(GYP): Remove "-pthread" from ldflags.
+  # Remove "-pthread" from ldflags.
   # -nodefaultlibs turns -pthread into a no-op, causing an unused argument
   # warning. Explicitly link with -lpthread instead.
+  configs -= [ "//build/config/compiler:pthread" ]
 
   libs = [
     "m",
   ]
 
   if (!is_mac) {
     libs += [
       "c",
       "gcc_s",
       "pthread",
@@ skipping 39 matching lines @@
       # Normally the generator takes care of RPATH. Our case is special because
       # the generator is unaware of the libc++.so dependency. Note that setting
       # RPATH here is a potential security issue. See the following for another
       # example of this issue: https://code.google.com/p/gyp/issues/detail?id=315
       "-Wl,-rpath,\$ORIGIN/",
     ]
   }
 
   lib_dirs = [ root_build_dir ]
 }