Use of size_t in Simulink legacy C code

I'm using Simulink's legacy_code tool to wrap C code into S-functions. This works fine, except when the legacy code uses the size_t type as part of its interface (i.e. as the type of one if its arguments or its return value). It seems that legacy_code does not support size_t (see the documentation). Here's the error message I'm getting:

The data type "size_t" is neither a built-in data type, nor defined by a Simulink.AliasType, Simulink.NumericType, Simulink.Bus object, or an enumerated data type in the base workspace.

Since the actual type of size_t depends on the compiler I would like to keep the function declaration I use with legacy_code portable. How can I achieve this?