Allow explicitly requiring universal deps

Previously, instructing Homebrew that all dependencies should be built
universal could be accomplished by redefining BuildOptions#universal? in
the class body:

  def build.universal?
    true
  end

However, the build object is no longer shared by all specs, so this is
insufficient. Instead, a new DSL method, "require_universal_deps", can
be used.

This feature is almost exclusively for wine, which requires universal
deps but does not itself have a universal option, since it is always
built 32-bit.

Library/Homebrew/build_options.rb

@@ -3,9 +3,10 @@ require 'options'
 # This class holds the build-time options defined for a Formula,
 # and provides named access to those options during install.
 class BuildOptions
-  attr_accessor :args
   include Enumerable
 
+  attr_accessor :args
+  attr_accessor :universal
   attr_reader :options
   protected :options
 
@@ -86,7 +87,7 @@ class BuildOptions
 
   # True if the user requested a universal build.
   def universal?
-    args.include?('--universal') && has_option?('universal')
+    universal || args.include?('--universal') && has_option?('universal')
   end
 
   # Request a 32-bit only build.

Library/Homebrew/formula.rb

@@ -777,6 +777,10 @@ class Formula
       @cc_failures << CompilerFailure.new(compiler, &block)
     end
 
+    def require_universal_deps
+      specs.each { |spec| spec.build.universal = true }
+    end
+
     def test &block
       return @test unless block_given?
       @test_defined = true