Use builtin libevent as last resort (or when asked)
README.md: document that the built in libevent may be used
README.md: mention --with-libevent=builtin
Merge branch 'eventually-use-builtin-libevent'

* eventually-use-builtin-libevent:
  README.md: mention --with-libevent=builtin
  README.md: document that the built in libevent may be used
  Use builtin libevent as last resort (or when asked)