Typically with libc implementations, wchar_t and co are either defined
entirely by the libc or, under libc's arrangement, by headers supplied
by the compiler.

Things like dlmalloc in relibc need these definitions from relibc itself
and that's already already furnished by relibc's stddef.h.

These additional definitions here are redundant and collide with
compiler headers - for example: onig_sys (something that uutils depends
on) breaks. Instead, this patch makes the compiler headers define
things appropriately.