build: Use $host_os variable to clarify configure.ac * configure.ac: Use $host_os instead of $canonical_host_type when operating system is being tested. * m4/acinclude.m4(OCTAVE_HOST_TYPE): Rename macro to OCTAVE_CANONICAL_HOST to be similar to Autoconf macro of same purpose. Guarantee that $host, $host_cpu, $host_vendor, and $host_os all have a value.