configure: set HAVE___ATTRIBUTE__ for heimdal

configure: set HAVE___ATTRIBUTE__ for heimdal

Without this, heimdal ends up defining __attribute__ away, causing
gcc-6 compile errors with -Werror=return-type because it can't tell
when functions have __attribute__((noreturn)).

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed Mar  9 13:52:26 CET 2016 on sn-devel-144