Allow meck to build under Erlang 17.0-rc2 and Erlang R16B03-1

I called the macro ERLANG_17 because they've stopped putting R in the string returned by `erlang:system_info(otp_release)`.

See [this thread](https://groups.google.com/forum/#!msg/erlang-programming/ySX5ZuSRjGU/QF-pVVnmYpoJ) about the change.