Don't mix external downloads with checked-in project source

When certain dependencies aren't found, OCIO's build scripts will download and build them automatically. (Yay!)

But then these files are left around intermixed with source, in the ext/ directory (which also contains sampleicc that is part of the OCIO repo). This is really inconvenient for several reasons: After a build, you have some things in ext that are part of the OCIO source repo, but other things that are NOT. You can't easily get rid of the temporarily downloads with `rm -rf ext`, and also you have to be super careful with `git commit -a` lest you accidentally sweep up those external projects along with whatever you are intentionally changing in OCIO's code.

I would suggest considering some subset of the following grab-bag of ideas to help address this:

* Have one subdirectory for external projects that are purposely embedded in the OCIO repo (like sampleicc), and a *separate* area for things you download but do not wish to become part of the OCIO project. Make sure you can easily `rm -r` the temp downloads just as easily and harmlessly as you can blow away the "build" and "dist" areas.

* Change the build scripts to put the downloads and source of the auto-build external packages in the "build" area, so it's not intermixed anywhere near the source.

* Add to `.gitignore` all the things that might be temporary downloads, so that a `git commit -a` can't possibly sweep them up, and even a `git status` isn't all cluttered with things that aren't part of the project.