libidn: build universal

libidn is a dependency of the GnuTLS 3.x series, and even though we
probably won't move to 3.x anytime soon, it's good to get the
dependencies up to snuff and work out any problems that might arise.

I also added a test of the executable while I was here.

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

Formula/libidn.rb

@@ -8,9 +8,14 @@ class Libidn < Formula
   depends_on 'pkg-config' => :build
 
   def install
+    ENV.universal_binary
     system "./configure", "--disable-dependency-tracking",
                           "--prefix=#{prefix}",
                           "--disable-csharp"
     system "make install"
   end
+
+  def test
+    system "#{bin}/idn --version"
+  end
 end