Fix build.py to work on Linux once again.
* strip DOS end-of-line characters from build.py for consistency with the rest of the source tree.
* use shutil.copy() instead of shutil.copyfile(). On Unix systems we must preserve execute permissions.
* add code to explicitly delete and recreate the target directory.
* Move build/jaxlib/__init_py to jaxlib/__init__.py and have the script move it into position, so the output directory for the jaxlib is an empty directory that the script creates.
