GnuPG - add support for patented IDEA cipher

Based on a patch from larseggert.

Formula/gnupg.rb

@@ -1,21 +1,48 @@
 require 'formula'
 
+class GnupgIdea <Formula
+  head 'http://www.gnupg.dk/contrib-dk/idea.c.gz', :using  => NoUnzipCurlDownloadStrategy
+  md5 '9dc3bc086824a8c7a331f35e09a3e57f'
+end
+
 class Gnupg <Formula
-  @url='ftp://ftp.gnupg.org/gcrypt/gnupg/gnupg-1.4.10.tar.bz2'
-  @homepage='http://www.gnupg.org/'
-  @sha1='fd1b6a5f3b2dd836b598a1123ac257b8f105615d'
+  url 'ftp://ftp.gnupg.org/gcrypt/gnupg/gnupg-1.4.10.tar.bz2'
+  homepage 'http://www.gnupg.org/'
+  sha1 'fd1b6a5f3b2dd836b598a1123ac257b8f105615d'
+
+  def options
+    [["--idea", "Build with (patented) IDEA cipher"]]
+  end
 
   def install
-    system "./configure", "--prefix=#{prefix}", "--disable-debug", "--disable-dependency-tracking", "--disable-asm"
+    if ARGV.include? '--idea'
+      opoo "You are building with support for the patented IDEA cipher."
+      d=Pathname.getwd
+      GnupgIdea.new.brew { (d+'cipher').install Dir['*'] }
+      system 'gunzip', 'cipher/idea.c.gz'
+    end
 
+    system "./configure", "--disable-debug", "--disable-dependency-tracking",
+                          "--prefix=#{prefix}",
+                          "--disable-asm"
     system "make"
     system "make check"
 
-    # amazingly even the GNU folks can bugger up their Makefiles, so we need
-    # to create these directories because the install target has the
+    # we need to create these directories because the install target has the
     # dependency order wrong
     bin.mkpath
     (libexec+'gnupg').mkpath
     system "make install"
   end
+
+  def caveats
+    if ARGV.include? '--idea'
+      <<-EOS.undent
+        Please read http://www.gnupg.org/faq/why-not-idea.en.html before doing so.
+        You will then need to add the following line to your ~/.gnupg/gpg.conf or
+        ~/.gnupg/options file:
+          load-extension idea
+      EOS
+    end
+  end
 end