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

configure.ac

@@ -54,8 +54,9 @@ AC_CHECK_SIZEOF(unsigned long)
 AC_CHECK_SIZEOF(unsigned long long)
 AC_CHECK_SIZEOF(int *)
 
-dnl Require compiler with C++11 support, do not use GNU extensions.
-AX_CXX_COMPILE_STDCXX_11(noext, mandatory)
+dnl Require compiler with C++11 support.
+dnl Allow GNU extensions to work around MinGW bugs around POSIX compatibility.
+AX_CXX_COMPILE_STDCXX_11(ext, mandatory)
 
 dnl some semi complex check for sys/socket so it works on darwin as well
 AC_CHECK_HEADERS([stdlib.h sys/types.h])

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"