Use S_ISDIR()/S_ISREG() instead of S_IFDIR/S_IFREG

[cvanaret]

file_kind() tests file type with the raw S_IFDIR/S_IFREG constants. On FreeBSD those constants are declared only under #if __XSI_VISIBLE in <sys/stat.h>, whereas the S_ISDIR()/S_ISREG() macros are defined unconditionally.

How to reproduce the error:

// fk.c
#include <sys/types.h>
#include <sys/stat.h>

static int file_kind(const char* name) {
    struct stat sb;
    if (stat(name,&sb)) return 0;
    if (sb.st_mode & S_IFDIR) return 2;   // raw constant
    if (sb.st_mode & S_IFREG) return 1;
    return 0;
}

int main(void) {
  return file_kind(".");
}

then:

clang -std=c99 -D_POSIX_C_SOURCE=200809L fk.c -o /dev/null
# fk.c:6:22: error: use of undeclared identifier 'S_IFDIR'
# fk.c:7:22: error: use of undeclared identifier 'S_IFREG'

See JuliaPackaging/Yggdrasil#13950