Automatically detect all cores when the --use-all-cores flag is passed

[ghost]

As title says - I decided not to pass the custom flags for building bindings, because I know sometimes multithread compilation sometimes can break stuff - if I will get free time I will test it with it too.

[asedeno]

GNU getopt is great for this, but BSD getopt doesn't do long options, and we want this to be portable. Bash's getopts builtin also doesn't seem to do long options without a lot of coaxing. Thoughts on a short option for this? I'm thinking -j, though I have reservations since it would not behave like the -j that we're passing to make, even though it is influencing it.