build:wafsamba: Enable feature-compatible declaration for Waf 1.8

[github at samba.org]

New comment by ddiss on Samba Github repository

https://github.com/samba-team/samba/pull/16#issuecomment-147359432
Comment:
Looks fine to me too - thanks Thomas!
Reviewed-by: David Disseldorp <ddiss at samba.org>