freetds: Use :optional

Closes Homebrew/homebrew#17496.

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

Formula/freetds.rb

@@ -5,10 +5,8 @@ class Freetds < Formula
   url 'http://mirrors.ibiblio.org/freetds/stable/freetds-0.91.tar.gz'
   sha1 '3ab06c8e208e82197dc25d09ae353d9f3be7db52'
 
-  option 'with-unixodbc', "Compile against unixODBC"
-
   depends_on "pkg-config" => :build
-  depends_on "unixodbc" if build.include? "with-unixodbc"
+  depends_on "unixodbc" => :optional
 
   def install
     args = %W[--prefix=#{prefix}