freetds: add flag for Kerberos support

Closes Homebrew/legacy-homebrew#28970.

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

Formula/freetds.rb

@@ -17,6 +17,7 @@ class Freetds < Formula
   option "enable-msdblib", "Enable Microsoft behavior in the DB-Library API where it diverges from Sybase's"
   option "enable-sybase-compat", "Enable close compatibility with Sybase's ABI, at the expense of other features"
   option "enable-odbc-wide", "Enable odbc wide, prevent unicode - MemoryError's"
+  option "enable-krb", "Enable Kerberos support"
 
   depends_on "pkg-config" => :build
   depends_on "unixodbc" => :optional
@@ -48,6 +49,10 @@ def install
       args << "--enable-odbc-wide"
     end
 
+    if build.include? "enable-krb"
+      args << "--enable-krb5"
+    end
+
     ENV.universal_binary if build.universal?
     system "./configure", *args
     system 'make'