Congratulations, you lied to the the compiler and it bit you.
Casts tell the compiler that you are intentionally breaking the rules and imply that you know what you are doing. C is happy to allow you to shoot yourself in the proverbial foot when you do this.
What you have with int const is in fact not a constant but a const qualified object. So if you play games of changing this object through pointer manipulations this might be possible but delivers undefined behavior.
The only integer constants in C are number tokens, integer character constants such as 'a' and enum constants.
There's not a while lot of mystery here. By using casts to tell the compiler that what you're changing isn't const qualified, you're causing undefined behavior:
6.7.3/5 "Type qualifiers" (C99):
If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined.
Some implementations might have placed the variable z in read only memory and you'd either get no apparent change or some sort of access violation.
In any case, undefined behavior means all bets are off - in your case you're able to see the apparent modification of a const value.