roaring.[ch]: Add attribution and licensing information
OS distributions that take copyright and licensing seriously will tend to become concerned about source files that appear to come from a third party and do not come with copyright/licensing information, and stating that these files are modified is required by clause 4(b) of the Apache license.
Include a copy of the Apache license
4(a) requires giving recipients of the work a copy of the license, and in any case it's best for source code distributions to be self-contained (including the full text of all applicable licenses).