Some warning fixes on modern compilers.

[dkorolev]

Hi @mzhurovich -- just a few tweaks for the latest (ubuntu stable) compilers:

$ g++ --version
g++ (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ clang++ --version
clang version 10.0.0-4ubuntu1 
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

Zero warnings for make individual_tests on either of them.

[dkorolev]

Closing this one in favor of https://github.com/C5T/Current/pull/887, which contains these changes.