Merge pull request #299 from cstrouse/add-libffi

Add package for libffi

packages/libffi.rb

+require 'package'
+
+class Libffi < Package
+  version '3.0.13'
+  source_url 'ftp://sourceware.org/pub/libffi/libffi-3.0.13.tar.gz'
+  source_sha1 'f5230890dc0be42fb5c58fbf793da253155de106'
+
+  depends_on 'autoconf'
+  depends_on 'automake'
+
+  def self.build
+    system "./configure", "--disable-debug", "--disable-dependency-tracking"
+  end
+
+  def self.install
+    system "make", "install"
+  end
+end