ENV/std: fully extend from HOMEBREW_PATH.

Rather than just re-adding HOMEBREW_PREFIX/bin if it's missing re-add
everything from HOMEBREW_PATH. This works well with or without
environment filtering being enabled but with environment filtering it
ensures that ENV/std just builds on the original user environment.

Library/Homebrew/extend/ENV/std.rb

@@ -9,15 +9,12 @@ module Stdenv
   SAFE_CFLAGS_FLAGS = "-w -pipe".freeze
   DEFAULT_FLAGS = "-march=core2 -msse4".freeze
 
-  def self.extended(base)
-    return if ORIGINAL_PATHS.include? HOMEBREW_PREFIX/"bin"
-    base.prepend_path "PATH", "#{HOMEBREW_PREFIX}/bin"
-  end
-
   # @private
   def setup_build_environment(formula = nil)
     super
 
+    PATH.new(ENV["HOMEBREW_PATH"]).each { |p| prepend_path "PATH", p }
+
     # Set the default pkg-config search path, overriding the built-in paths
     # Anything in PKG_CONFIG_PATH is searched before paths in this variable
     self["PKG_CONFIG_LIBDIR"] = determine_pkg_config_libdir