Use SSIZE_T for ssize_t on Windows (#5811)

The PR adds a compile assert to ensure that size_t and ssize_t
have the same size, just to be sure.

* Use SSIZE_T for ssize_t on Windows

* Use SSIZE_T and SSIZE_T_MAX

* Add missing header

* Exclude MinGW from ssize_t ifdefs

src/H5.c

@@ -154,6 +154,9 @@ H5_init_library(void)
      */
     H5_INIT_GLOBAL = true;
 
+    /* Make sure we picked a good type for ssize_t if it wasn't present */
+    HDcompile_assert(sizeof(size_t) == sizeof(ssize_t));
+
 #ifdef H5_HAVE_PARALLEL
     {
         int mpi_initialized;

src/H5private.h

@@ -371,7 +371,7 @@
  * Only needed where ssize_t isn't a thing (e.g., Windows)
  */
 #ifndef SSIZE_MAX
-#define SSIZE_MAX ((ssize_t)(((size_t)1 << (8 * sizeof(ssize_t) - 1)) - 1))
+#define SSIZE_MAX SSIZE_T_MAX
 #endif
 
 /*

src/H5public.h

@@ -289,21 +289,11 @@ typedef int htri_t;
  *
  * Use of ssize_t should be discouraged in new code.
  */
-#if H5_SIZEOF_SSIZE_T == 0
-/* Undefine this size, we will re-define it in one of the sections below */
-#undef H5_SIZEOF_SSIZE_T
-#if H5_SIZEOF_SIZE_T == H5_SIZEOF_INT
-typedef int ssize_t;
-#define H5_SIZEOF_SSIZE_T H5_SIZEOF_INT
-#elif H5_SIZEOF_SIZE_T == H5_SIZEOF_LONG
-typedef long ssize_t;
-#define H5_SIZEOF_SSIZE_T H5_SIZEOF_LONG
-#elif H5_SIZEOF_SIZE_T == H5_SIZEOF_LONG_LONG
-typedef long long ssize_t;
-#define H5_SIZEOF_SSIZE_T H5_SIZEOF_LONG_LONG
-#else /* Can't find matching type for ssize_t */
-#error "nothing appropriate for ssize_t"
-#endif
+#if defined(_WIN32) && !defined(__MINGW32__)
+#include <BaseTsd.h>
+typedef SSIZE_T ssize_t;
+/* This will have been defined to 0 in configure */
+#define H5_SIZEOF_SSIZE_T H5_SIZEOF_SIZE_T
 #endif
 
 /**