Don't change files that are committed to the repo during build

[sirpalee]

We are building multiverse through rez, where the release process checks for outstanding changes in the repo, before doing the actual release. Right now, lib/Alembic/AbcCoreGit/milliways/config.h is not only regenerated during build, but it's also comitted to the repo.

Usually built time generated files are either generated into CMAKE_CURRENT_BINARY_DIR, or they are not part of the repo + are ignored by git. Is there any reason why the file has to be part of the repo?