heimdal: bump to version 1.6rc2

Helps drop some patches and hence autoreconf saving time.
Also allows parallel builds.

And fixes:
http://buildroot-busybox.2317881.n4.nabble.com/PATCH-v2-heimdal-explicitly-set-lpthread-td99422.html


without the need to hardcode libraries (disables/withouts weren't enough
though i've added them anyway for correctness).

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>