Inclue HOMEBREW_USE_CLANG in --env output

Library/Homebrew/cmd/--env.rb

@@ -26,7 +26,8 @@ module Homebrew extend self
     end
 
     %w[ CFLAGS CXXFLAGS CPPFLAGS LDFLAGS MACOSX_DEPLOYMENT_TARGET MAKEFLAGS PKG_CONFIG_PATH
-        HOMEBREW_DEBUG HOMEBREW_VERBOSE HOMEBREW_USE_GCC HOMEBREW_USE_LLVM HOMEBREW_SVN ].each do |k|
+        HOMEBREW_DEBUG HOMEBREW_VERBOSE HOMEBREW_USE_CLANG HOMEBREW_USE_GCC HOMEBREW_USE_LLVM
+        HOMEBREW_SVN ].each do |k|
       value = env[k]
       puts "#{k}: #{value}" if value
     end