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.

gzwrite.c

@@ -74,7 +74,7 @@ local int gz_comp(state, flush)
     int flush;
 {
     int ret;
-    ssize_t got;
+    z_ssize_t got;
     unsigned have;
     z_streamp strm = &(state->strm);