Adds missing AC_CANONICAL_{TARGET,SYSTEM} calls in configure.ac
to initialize ${target_os} and other variables. Without changes
${target_os} is always undefined (an empty string).
I reckon to add "requires libnl3-dev" instead of only libnl. I was trying to ./configure bmon for about 15 mins without success only because error message was showing me that my system is missing libnl not libnl3 :-(
