Only build hconfd if --enable-confd was passed

A later, more complete patch, will allow selecting between either the
Python version or the Haskell version. This is just a temporary
solution to help building without all the needed Haskell libraries.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>