link/link.c: In function ‘main’:
link/link.c:30:18: error: ‘symlink’ undeclared (first use in this function)
   30 |             fn = symlink;
      |                  ^~~~~~~
link/link.c:30:18: note: each undeclared identifier is reported only once for each function it appears in
make: *** [Makefile;31: link.o] Error 1

#if defined(__unix__)
    #include <unistd.h>

    #include <dirent.h>
    #include <fcntl.h>

    #include <sys/mman.h>
    #include <sys/random.h>
    #include <sys/stat.h>
    #include <sys/syscall.h>
    #include <sys/sysctl.h>
    #include <sys/sysinfo.h>
    #include <sys/termios.h>
    #include <sys/types.h>
    #include <sys/user.h>
#elif defined(_WIN32)
    ...
#endif // OS-Dependent modules

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
...

extern int symlink(const char* actualpath, const char* sympath);

extern int symlink (const char *__from, const char *__to)
     __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1, 2))) ;

extern int link (const char *__from, const char *__to)
     __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1, 2))) ;

#if defined(__unix__)
    #include <unistd.h>

    #include <dirent.h>
    #include <fcntl.h>
#elif defined(_WIN32)
    // Removed for brevity
#endif // OS-Dependent modules

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>

#if !defined(FALSE) || !defined(TRUE)
enum { FALSE, TRUE };
#endif // TRUE || FALSE

// Why does the compiler require this declaration, and only for symlink?
extern int symlink(const char* actualpath, const char* sympath);

typedef int (*link_fn)(const char* path, const char* link_path);

int main(int argc, char *argv[])
{
    if (argc == 2) {
        if (strcmp(argv[1], "--help") == 0) {
            printf("nUsage: %s <Existing Filename> <New Filename>nn", argv[0]);
            printf("Options: n");
            printf("  -s    Create symbolic link instead of a hard linknn");

            return EXIT_SUCCESS;
        }
    }

    if (argc < 3) {
        fprintf(stderr, "Usage: %s <Existing Filename> <New Filename>n", argv[0]);

        return EXIT_FAILURE;
    }

    link_fn fn = link;

    for (size_t i = 1; i < (size_t) argc - 2; ++i) {
        if (strcmp(argv[i], "-s") == 0) {
            fn = symlink;
        }
    }

    const char* existing_path = argv[argc - 2];
    const char* new_path      = argv[argc - 1];

    errno = 0;

    const int return_code = fn(existing_path, new_path);

    if (return_code == -1) {
        fprintf(stderr, "[Error] %sn", strerror(errno));

        return EXIT_FAILURE;
    }

    return EXIT_SUCCESS;
}

CC=gcc
CFLAGS="-std=c17 -Wall -Wextra -Werror -pedantic"

gcc version 9.1.0
Manjaro Linux 18.0.4
x86-64