system_config: don’t output std/superenv vars.

Library/Homebrew/extend/ENV/std.rb

@@ -13,6 +13,8 @@ module Stdenv
   def setup_build_environment(formula = nil)
     super
 
+    self["HOMEBREW_ENV"] = "std"
+
     PATH.new(ENV["HOMEBREW_PATH"]).each { |p| prepend_path "PATH", p }
 
     # Set the default pkg-config search path, overriding the built-in paths

Library/Homebrew/extend/ENV/super.rb

@@ -38,6 +38,7 @@ module Superenv
     super
     send(compiler)
 
+    self["HOMEBREW_ENV"] = "super"
     self["MAKEFLAGS"] ||= "-j#{determine_make_jobs}"
     self["PATH"] = determine_path
     self["PKG_CONFIG_PATH"] = determine_pkg_config_path

Library/Homebrew/system_config.rb

@@ -151,6 +151,7 @@ class SystemConfig
       if defaults_hash[:HOMEBREW_CACHE] != HOMEBREW_CACHE.to_s
         f.puts "HOMEBREW_CACHE: #{HOMEBREW_CACHE}"
       end
+      unless ENV["HOMEBREW_ENV"]
         ENV.sort.each do |key, value|
           next unless key.start_with?("HOMEBREW_")
           next if boring_keys.include?(key)
@@ -158,6 +159,7 @@ class SystemConfig
           value = "set" if key =~ /(cookie|key|token|password)/i
           f.puts "#{key}: #{value}"
         end
+      end
       f.puts hardware if hardware
       f.puts "Homebrew Ruby: #{describe_homebrew_ruby}"
       f.print "Clang: "