Add --use-llvm flag to build environment output.

bin/brew

@@ -69,6 +69,8 @@ EOS
 end
 
 def dump_build_env env
+  puts "\"--use-llvm\" was specified" if ARGV.include? '--use-llvm'
+
   %w[CC CXX LD CFLAGS CPPFLAGS CXXFLAGS LDFLAGS MAKEFLAGS HOMEBREW_USE_LLVM].each do |k|
     puts "#{k}: #{env[k]}"
   end