first point:

the provided clang installation appears to find a definition of strdup() which ought to return an int,

When the compiler encounters a function call for which it does not find a function declaration, it assumes the function has this prototype: int func_name ().
strdup() is sadly not yet part of standard C (it will be added in C23). This is why the compiler does not find it in <string.h> : it is a POSIX extension. you have to #define _GNU_SOURCE before #include <string.h> to make it visible:

#define _GNU_SOURCE
#include <string.h>

second point: your code is almost correct, but has 2 mistakes:

• you are not handling empty strings correctly,
• you are adding a trailing space at the end of the string

next time just use

 --> ``` 
    some code
again this  --> ```