Mac OS X build fails due to presence of BUILD file which prevents ./build/ directory from being created

[manics]

Using current master:

$ python setup.py bdist_wheel
running bdist_wheel
running build
running build_py
creating build/lib/tensorflow_privacy
error: could not create 'build/lib/tensorflow_privacy': Not a directory

OS X uses a case-insensitive filesystem, though it maintains the case when displaying names. The build process creates a ./build/ directory. This works on Linux since the filesystem is case-sensitive, but not on OS X. Locally renaming the file to BUILD.txt allows python setup.py bdist_wheel to succeed.