Windows: Use MinGW's stdio functions.

The non-standard ones from msvcrt.dll appear to work
most of the time with XZ Utils, but there are some
corner cases where things may go very wrong. So it's
good to use the better replacements provided by
MinGW(-w64) runtime.

src/common/sysdefs.h

@@ -24,6 +24,11 @@
 #	include <config.h>
 #endif
 
+// Get standard-compliant stdio functions under MinGW and MinGW-w64.
+#ifdef __MINGW32__
+#	define __USE_MINGW_ANSI_STDIO 1
+#endif
+
 // size_t and NULL
 #include <stddef.h>