Stdenv: Add ENV.libxml2 and ENV.x11 for Linux

Add ENV.libxml2 primarily for the use of test do blocks.
Add a dummy ENV.x11 function.

See Linuxbrew/brew#356 and Linuxbrew/brew#382

Library/Homebrew/extend/ENV/std.rb

@@ -210,6 +210,8 @@ module Stdenv
   end
   alias generic_set_cpu_flags set_cpu_flags
 
+  def x11; end
+
   # @private
   def effective_arch
     if ARGV.build_bottle?

Library/Homebrew/extend/os/extend/ENV/std.rb

@@ -1,2 +1,6 @@
 require "extend/ENV/std"
-require "extend/os/mac/extend/ENV/std" if OS.mac?
+if OS.mac?
+  require "extend/os/mac/extend/ENV/std"
+elsif OS.linux?
+  require "extend/os/linux/extend/ENV/std"
+end

Library/Homebrew/extend/os/linux/extend/ENV/std.rb

@@ -0,0 +1,7 @@
+module Stdenv
+  def libxml2
+    append "CPPFLAGS", "-I#{Formula["libxml2"].include/"libxml2"}"
+  rescue FormulaUnavailableError
+    nil
+  end
+end