Yasutaka ATARASHI

Thanks. I'll make a pull request.

I recognized MDB_VL32 feature has not been officially released yet. So, offical releases do not have `mdb_size_t`. We can, maybe, employ some metaprogramming technique to detect if the type is...

Sorry for the long hiatus. My use case was solved by another measure, so there is less concern for this issue... I made some work anyway; I created the PR....