Compiler warnings / errors from new logging feature on Windows

[shikokuchuo]

Describe the bug

1. Compiler warns of unused variable in win_clock.c
2. Some Windows platforms do not have timespec_get (MinGW) so we should test for it like on POSIX, and provide an alternative, or have an option to disable logging entirely.

** Environment Details **

• NNG 1.8.0
• Various Windows build machines
• GNU 10.4.0 under MinGW
• Static Lib

Additional context
(1) is an easy fix and I'll provide a PR.
(2) as I do not make use of logging, I can live with a CMake check and compile time define - but others may want something that works.

[gdamore]

Argh. MinGW again. I fricking hate that because it misses a lot of real Windows APIs. This one now too.

In the past I've said that I don't support it. I'm not sure what to do here. This needs to be at least a warning.

[gdamore]

So apparently MinGW-w64 has a timespec_get API if you use the Universal C Runtime (UCRT). The default legacy MSVCRT is lacking, and I have no interest in wasting my time with it.