[CMake] ExternalProject and redundant update/configure/compile actions.

[David Cole]

Thanks for the clarifications.

So things sped up because now you're getting incremental builds, 
whereas before, you would have to force full builds when the tag 
changed. That makes sense.

I guess I shouldn't be... but I'm always surprised that people even 
attempt incremental rebuilds of large-scale systems. I don't trust all 
the tools 100% to produce correct results when making incremental 
changes in between builds. There are too many links in the chain and 
too many things that can go wrong. Too many wasted days tracking down 
mysteries that ended up being simple stale out-of-date build products 
in my past I guess.

Rather than giving up on incremental builds, I suppose I should just 
adopt a new personal policy: *if* something goes mysteriously wrong, 
first try a full re-build, ... *then* investigate if necessary. :-)


Thx,
David