wscript: With --enable-$x, the existence of $x should be mandatory If the user says they want to compile aubio with $x enabled, it should be a critical failure if aubio can't do that. If the user only wants to compile aubio with $x enabled if it's possible, then they shouldn't be setting `--enable-$x`. With this change, aubio defaults to "include $x if we find it", which the user can override with either --enable-$x or --disable-$x.