Make waf fail if submodules are out of date.

Make waf fail if submodules are out of date.

Instead, suggest the user run 'git submodule update'.

This should prevent users from accidentally building Samba against
outdated or too new versions of the bundled third party libraries
after switching branches.

I've opted to make this an error rather than actually
running 'git submodule update' directly, as the latter could
cause unpredictable behaviour. If we find that manually updating
submodules is too cumbersome, we can always change this. The normal mode
of operation for developers should not involve any submodules at all,
but system versions of these libraries.

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>