Make stdenv handling less intrusive.

* Don't encode "-Qunused-arguments", as this is only supported in Clang
  and will break building when C/C++ compiler is switched during the
  process, e.g. building gcc. The warning is harmless without
  "-Werror".

Library/Homebrew/extend/ENV/std.rb

+# -*- coding: utf-8 -*-
 require 'hardware'
 require 'os/mac'
 require 'extend/ENV/shared'
@@ -195,7 +196,6 @@ module Stdenv
     replace_in_cflags(/-Xarch_#{Hardware::CPU.arch_32_bit} (-march=\S*)/, '\1')
     # Clang mistakenly enables AES-NI on plain Nehalem
     set_cpu_cflags '-march=native', :nehalem => '-march=native -Xclang -target-feature -Xclang -aes'
-    append_to_cflags '-Qunused-arguments'
     @compiler = :clang
   end