Comment by Bilirubino

As I commented above, Int32 and Int64 are part of the standard library since at least 4.X Ocaml versions (we are now in 5.3). So normally all them are available when you install any distribution of Ocaml. Note that there is also a type named nativeint (which, I think is the kind of int that you were looking for in all your comments and post) and it is part of the standard library, so in summary:

Int type (the one you dislike for systems programming)

Int32 type (part of the standard library, one of those you were looking for)

Int64 type (part of the standard library, one of those you were looking for)

Nativeint (part of the standard library, maybe the one you were looking for)

The library stdint is other option, which can be convenient in some cases but for Int32 and Int64 you don't need it also for Nativeint you don't need it.