Use a struct as second parameter to be future proof (and also allows to
specify default values for the parameters)
Closes#2679 as it was just a matter of a few lines of code.
It had the downside of running all the comptime blocks and resolving
all the usingnamespaces of each system, when just trying to discover if
the current system is a particular one.
For Darwin, where it's nice to use `std.Target.current.isDarwin()`, this
demonstrates the utility that #425 would provide.
