Update guile dependencies.

Formula/guile.rb

@@ -5,12 +5,13 @@ class Guile <Formula
   @homepage='http://www.gnu.org/software/guile/'
   @sha1='c8d1d25ed413b48493ec5b0cbf4de8593cab4a21'
 
+  depends_on 'pkg-config'
+  depends_on 'libffi'
   depends_on 'libunistring'
   depends_on 'bdw-gc'
   depends_on 'gmp'
   
-  # GNU Readline is required
-  # libedit won't work.
+  # GNU Readline is required; libedit won't work.
   depends_on 'readline'
 
   def install
@@ -18,11 +19,6 @@ class Guile <Formula
         "--prefix=#{prefix}", 
         "--disable-debug", 
         "--disable-dependency-tracking",
-        # Specifically look for readline here
-        # At least, we don't want the fake readline in
-        # /usr/lib to trump us, since it doesn't export
-        # all the same symbols
-        # --adamv
         "--with-libreadline-prefix=#{Formula.factory('readline').prefix}"
     system "make install"
   end