Fix issue with extension check

[klezVirus]

Hi man, the syscall version was not working because the file check against the binary was not working when compiled with MSVC. Indeed, when MSVC is used, the unistd function strcasecmp is replaced by stricmp. However, completely unexpectedely, while strcasecmp return 0 when the two strings are equal, the custom implementation stricmp return 0 when they are different. Indeed, following through the definition in donut.h:

#if defined(_MSC_VER)
#pragma comment(lib, "advapi32.lib")
#pragma comment(lib, "user32.lib")
#define strcasecmp stricmp

And if we go and ivestigate in in clib.c, we can see that the stricmp implementation is:

int stricmp(const char *str1, const char *str2) {
    while (*str1 && *str2) {
      if ((*str1 | 0x20) != (*str2 | 0x20)) {
        return 0;
      }
      str1++; str2++;
    }
    return *str2 == 0;
}

Which evidentely return 0 if the two strings are different.

[klezVirus]

This is just a workaround for MSVC, actually. I think it would be better to change the stricmp implementation and re-change all of its occurrencies in the code when required.