message: doc: Update OpenBSD build guide for 6.4
diff: OpenBSD build guide
  ======================
  -(updated for OpenBSD 6.3)
  +(updated for OpenBSD 6.4)
  This guide describes how to build bitcoind and command-line utilities on OpenBSD.
  @@ -14,7 +14,7 @@ Run the following as root to install the base dependencies for building:
  ```bash
  pkg_add git gmake libevent libtool boost
  pkg_add autoconf # (select highest version, e.g. 2.69)
  -pkg_add automake # (select highest version, e.g. 1.15)
  +pkg_add automake # (select highest version, e.g. 1.16)
  pkg_add python # (select highest version, e.g. 3.6)
  git clone https://github.com/bitcoin/bitcoin.git
  @@ -60,8 +60,8 @@ Preparation:
  export AUTOCONF_VERSION=2.69
  # Replace this with the automake version that you installed. Include only
  -# the major and minor parts of the version: use "1.15" for "automake-1.15.1".
  -export AUTOMAKE_VERSION=1.15
  +# the major and minor parts of the version: use "1.16" for "automake-1.16.1".
  +export AUTOMAKE_VERSION=1.16
  ./autogen.sh
  ```
  