formula_installer: warn on deprecated options.

Library/Homebrew/formula_installer.rb

@@ -150,6 +150,12 @@ class FormulaInstaller
       raise "Unrecognized architecture for --bottle-arch: #{arch}"
     end
 
+    f.active_spec.deprecated_flags.each do |deprecated_option|
+      old_flag = deprecated_option.old_flag
+      new_flag = deprecated_option.current_flag
+      opoo "#{f.name}: #{old_flag} is deprecated; use #{new_flag} instead!"
+    end
+
     oh1 "Installing #{Tty.green}#{f.name}#{Tty.reset}" if show_header?
 
     @@attempted << f