configure: Upgrade a few trivial macros to 2.69. [skip appveyor]

configure: Upgrade a few trivial macros to 2.69. [skip appveyor]

Let autoupdate 2.69 add/remove some macro argument quotes as it sees
fit, replace AC_CANONICAL_SYSTEM with AC_CANONICAL_HOST (autoupdate
replaces it with AC_CANONICAL_TARGET by default, but this project uses
only the host_os and host_cpu shell variables, so, consistently with
tcpdump and tcpreplay, let's not overshoot it).  Replace AC_WARN with
AC_MSG_WARN, add an explicit tag to AC_CONFIG_COMMANDS and convert
AC_OUTPUT.