> Why match the C standard at all?

The referenced header file is defined thusly:

  #include <stddef.h>
  #include <stdbool.h>
  
  void *malloc(size_t size);
  void *calloc(size_t n, size_t size);
  void *realloc(void *ptr, size_t size);
  void free(void *ptr);

These function declarations are equivalent to those defined by the C standard due their being "drop-in" replacements. Therefore, reproducing same is unneeded.

> I'm in a position to do this in my programming language project. Wrote my own allocator for it. Maybe it's time to reinvent a better wheel.

Wonderful.

But if your intent is to replace the aforementioned C standard library memory allocation functions, then they would have to have the same signatures of the functions being replaced. Which leads back to the original assertion that there is no need for a header file which declares the same C functions defined by the C standard library for which they replace.