ruby: autoconf dep for HEAD on Xcode 4.3

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

Formula/ruby.rb

@@ -7,6 +7,7 @@ class Ruby < Formula
 
   head 'http://svn.ruby-lang.org/repos/ruby/trunk/'
 
+  depends_on 'autoconf' => :build if MacOS.xcode_version.to_f >= 4.3 and ARGV.build_head?
   depends_on 'pkg-config' => :build
   depends_on 'readline'
   depends_on 'gdbm'
@@ -44,7 +45,7 @@ class Ruby < Formula
       exit 1
     end
 
-    system "autoconf" unless File.exists? 'configure'
+    system "autoconf" if ARGV.build_head?
 
     args = ["--prefix=#{prefix}",
             "--enable-shared"]