[Distutils] upgrading buildout

[whit morriss]

I'm perhaps running buildout in a bit of unusual fashion (from my 
virtual env bin rather than the builds bin) and updated it recently from 
1.0.2 to 1.1.0 to debug some issue folks were having with fresh 
checkouts of one of my builds (see previous message).

I was suprised when all my zc.recipe.cmmi deps were recompiled. not a 
huge deal due to the unusual circumstances, but seemed avoidable.

I realize my upgrade is dependent on my virtualenv setup and was 
wondering if there is some standard way to upgrade a build's buildout 
version while avoiding unnecessary rebuilding.

-w