Update mpqfs to fix DOS support

[AJenbo]

Fixes #8537

[StephenCWills]

I don't know if _POSIX_C_SOURCE doesn't work for DJGPP because the platform isn't fully POSIX compliant or what, but fdopen appears to be declared as a compiler extension rather than a POSIX extension. So I just went ahead and enabled compiler extensions specifically for DOS.

[AJenbo]

I don't know if _POSIX_C_SOURCE doesn't work for DJGPP because the platform isn't fully POSIX compliant or what, but fdopen appears to be declared as a compiler extension rather than a POSIX extension. So I just went ahead and enabled compiler extensions specifically for DOS.

Enabling the extensions did solve the reported issue, but I tried actually building things this time (the previous fix was a bit hasty) and there where some additional issues so I ended up fixing both in mpqfs anyway.

diasurgical/mpqfs@293b3a3