ghc: use options DSL

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/ghc.rb

@@ -16,7 +16,7 @@ end
 class Ghc < Formula
   homepage 'http://haskell.org/ghc/'
   version '7.4.2'
-  if Hardware.is_64_bit? and not ARGV.build_32_bit?
+  if Hardware.is_64_bit? and not build.build_32_bit?
     url 'http://www.haskell.org/ghc/dist/7.4.2/ghc-7.4.2-x86_64-apple-darwin.tar.bz2'
     sha1 '7c655701672f4b223980c3a1068a59b9fbd08825'
   else
@@ -26,6 +26,8 @@ class Ghc < Formula
 
   depends_on NeedsSnowLeopard.new
 
+  option '32-bit'
+
   # Avoid stripping the Haskell binaries & libraries.
   # See: http://hackage.haskell.org/trac/ghc/ticket/2458
   skip_clean ['bin', 'lib']
@@ -38,10 +40,6 @@ class Ghc < Formula
       EOS
   end
 
-  def options
-    [['--32-bit', 'Build 32-bit only.']]
-  end
-
   def install
     system "./configure", "--prefix=#{prefix}"
     system "make install"