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

[lgritz]

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.