First, you are not forced to use intmax_t (unless of course you inherit it from someone else).

Second, in my personal opinion, it's really much safer to use C89/C90, unless you have very good reasons to use the extensions present in the newer versions of the standard. Even large projects like SQLite appreciate this approach. And if you are developing for an environment supporting C99 and newer standards, you could as well use C++ (or a newer language).