enable s3 waf build in autobuild

[tridge at samba.org]

Hi All,

Does anyone have any objections to us enabling the source3 waf build
in autobuild? I'm thinking of making it do a build, but not run tests.

The reason for enabling it now, is that it shares its build rules with
the top level s3/s4 merged build. Ensuring that the s3 waf build keeps
working will make life much easier for the current merge work.

This would mean that all committers will need to fix any build issues
with the source3 waf build that may arise on sn-devel, or autobuild
will fail for your commit. 

We will still have the s3 autoconf build as well, so that means all
committers will need to ensure any build rule changes are made in 2
places for code that affects s3.

The s3 waf build has been in the build farm for a few weeks now, and I
think it is now reasonable to think of it as a "should not be broken"
build.

Cheers, Tridge