New wrapper macro for printf("%zu"), for old VS compat.

A user reports that Visual Studio 2013 and earlier have printf
implementations in their C library that don't support the 'z' modifier
to indicate that an integer argument is size_t. The 'I' modifier
apparently works in place of it.

To avoid littering ifdefs everywhere, I've invented my own inttypes.h
style macros to wrap size_t formatting directives, which are defined
to %zu and %zx normally, or %Iu and %Ix in old-VS mode. Those are in
defs.h, and they're used everywhere that a %z might otherwise get into
the Windows build.

defs.h

@@ -22,9 +22,16 @@
 #define PRIdMAX "I64d"
 #define PRIXMAX "I64X"
 #define SCNu64 "I64u"
+#define SIZEx "Ix"
+#define SIZEu "Iu"
 uintmax_t strtoumax(const char *nptr, char **endptr, int base);
 #else
 #include <inttypes.h>
+/* Because we still support older MSVC libraries which don't recognise the
+ * standard C "z" modifier for size_t-sized integers, we must use an
+ * inttypes.h-style macro for those */
+#define SIZEx "zx"
+#define SIZEu "zu"
 #endif
 
 #if defined __GNUC__ || defined __clang__