[osg-submissions] New manifest fix to CMake

[Roger James]

> > the users to install the MS redistributables. In this case he will need
> to
> > put it to OFF and make sure to embed the redistributables with its
> > application.
> 
[Roger James] 

I am afraid my vote is to keep it on. Manifests are a fact of life on
windows nowadays and are not going to go away.

I tried long and hard to avoid putting the ms redist exe in our
distributions but gave in, in the end; one reason being that I was not sure
of the legality of redistributing the ms dlls without the licence which is
bound up in the exe with them.

Roger