Makefile: prevent bogus make targets

This is a kludge.  Add variable CONTRIBUTORS_URL so that no url is
lexically within the "herefile" AUTHORS_HEADER.  Otherwise https and
everything else preceding the colon would be considered a make target.
Makefile: absorb .mailmap
Makefile: use printf instead of echo
Makefile: cosmetics
Makefile: add new .mailmap rule
AUTHORS.md: update list of contributors
Merge branch 'authors'