bdw-gc: add --universal option

Closes Homebrew/homebrew#13443.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>

Formula/bdw-gc.rb

@@ -5,7 +5,12 @@ class BdwGc < Formula
   url 'http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gc-7.2.tar.gz'
   md5 'd17aecedef3d73e75387fb63558fa4eb'
 
+  def options
+    [['--universal', 'Make a 32/64-bit Intel build.']]
+  end
+
   def install
+    ENV.universal_binary if ARGV.build_universal?
     system "./configure", "--disable-debug",
                           "--disable-dependency-tracking",
                           "--prefix=#{prefix}",