Add ENOTSUP for linux, which maps to EOPNOTSUPP

glibc aliases ENOTSUP to EOPNOTSUPP. 

Therefore declare ENOTSUP as EOPNOTSUPP on linux platforms as well.

This usually works and should work in D as well:
```C
#include <errno.h>
#include <stdio.h>

int main(int argc, char * argv[]) {
    printf("ENOTSUP: %d\n", ENOTSUP);
    printf("EOPNOTSUPP: %d\n", EOPNOTSUPP);
    return 0;
}
```