re2:replace changes

You may or may not want to take the "Return original string" change. It makes re2:replace better match re:replace's behavior, but it is an API change and I can also see an argument for the current behavior.

The Makefile change I made because "make test" was failing for me after my re2:replace changes because rebar was picking up a system version of the re2 library instead of this one. It introduces some GNU-makeisms.