build: Make --disable-g13 the default.

* Makefile.am (DISTCHECK_CONFIGURE_FLAGS): Add --enable-g13.  Remove
--enable-gpgtar because that is enabled anyway.
* configure.ac: Do not build g13 by default.

The g13 part is not very useful for a standard user right now, thus do
not build it.

• Signed-off-by: Werner Koch <wk@gnupg.org>