gsl: add --universal option

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/gsl.rb

@@ -5,8 +5,14 @@ class Gsl < Formula
   homepage 'http://www.gnu.org/software/gsl/'
   md5 'd55e7b141815412a072a3f0e12442042'
 
+  def options
+    [["--universal", "Build a universal binary."]]
+  end
+
   def install
-    system "./configure", "--prefix=#{prefix}", "--disable-dependency-tracking"
+    ENV.universal_binary if ARGV.include? "--universal"
+
+    system "./configure", "--disable-dependency-tracking", "--prefix=#{prefix}"
     system "make" # A GNU tool which doesn't support just make install! Shameful!
     system "make install"
   end