It's trivial to roll your own basic <stdint.h> with those types for existing C89 implementations.

Unless they don't provide the corresponding type natively at all (which can be the case for some 16-bit platforms and 64-bit). But then you have a bigger problem anyway.

Only with effort Linux was able to be ported to Clang. All compilers that compile Linux should also be able to deal with size_t and consorts.