igraph: use options dsl

Formula/igraph.rb

@@ -5,15 +5,13 @@ class Igraph < Formula
   url 'http://downloads.sourceforge.net/sourceforge/igraph/igraph-0.6.tar.gz'
   sha1 'd9a6e7c0462b5948ebfbf9f1d2685141f5867fe3'
 
-  # GMP is optional, and doesn't have a universal build
-  depends_on 'gmp' => :optional unless ARGV.build_universal?
+  option :universal
 
-  def options
-    [["--universal", "Build a universal binary."]]
-  end
+  # GMP is optional, and doesn't have a universal build
+  depends_on 'gmp' => :optional unless build.universal?
 
   def install
-    ENV.universal_binary if ARGV.build_universal?
+    ENV.universal_binary if build.universal?
 
     system "./configure", "--disable-debug",
                           "--disable-dependency-tracking",