* configure.ac: Expand the ever-growing list of ruby binaries to look for. Add versions up to ruby 2.4. Add binary names used by Ruby ports on *BSD. Prompted by a patch which was added to OpenBSD ports to make SVN detect current versions of Ruby.
Build against the system utf8proc library by default instead of the internal copy of the utf8proc code. Add --with-utf8proc=PREFIX|internal to allow the user to specify either where to find utf8proc or that the internal copy be used. The Windows build will continue to use the internal utf8proc code.