Allow GNU extensions for GCC to avoid MinGW bugs

Most MinGW versions work correctly with and without _POSIX_C_SOURCE
or XOPEN macros [1] and don't need any special handling, but some
installations refuse to work correctly.

To be more precise: *some* MinGW installations turn off POSIX support
after turning on -std=c++11 WITHOUT -ansi -Wpedantic, -pedantic-errors
and similar.  This behaviour is inconsistent between various Windows
versions and MinGW distributions and manifests itself only when using
GCC (Clang behaves correctly).

[1] https://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html

src/dos/drive_local.cpp

@@ -19,16 +19,16 @@
 // Uncomment to enable file-open diagnostic messages
 // #define DEBUG 1
 
-#include <stdio.h>
+#include "drives.h"
+
+#include <cstdio>
 #include <stdlib.h>
 #include <string.h>
 #include <time.h>
 #include <errno.h>
 
-#include "dosbox.h"
 #include "dos_inc.h"
 #include "dos_mscdex.h"
-#include "drives.h"
 #include "support.h"
 #include "cross.h"
 #include "inout.h"