s4:heimdal_build: define HAVE_GSS_KRB5_CRED_{SKIP_TRANSIT_CHECK,ITERATE_ACCEPTOR_KEYT...

s4:heimdal_build: define HAVE_GSS_KRB5_CRED_{SKIP_TRANSIT_CHECK,ITERATE_ACCEPTOR_KEYTAB}_X

We can only do that for our own copy of heimdal, see
https://github.com/heimdal/heimdal/pull/656

In future we may want to use
source4/heimdal_build/wscript_configure only for
our in tree copy of heimdal and do real configure
checks for the system heimdal build.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12907
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14125

Signed-off-by: Stefan Metzmacher <metze@samba.org>