Create z_size_t and z_ssize_t types.

Normally these are set to size_t and ssize_t. But if they do not
exist, then they are set to the smallest integer type that can
contain a pointer. size_t is unsigned and ssize_t is signed.

gzread.c

@@ -23,7 +23,7 @@ local int gz_load(state, buf, len, have)
     unsigned len;
     unsigned *have;
 {
-    ssize_t ret;
+    z_ssize_t ret;
 
     *have = 0;
     do {