C++17 introduction and minor clean-ups

[markuspg]

C++17 is default with GCC 11 (released in April 2021) and LLVM 16 (released in March 2023). It's fully covered since GCC 8 (released in May 2018) and LLVM 5 (released in September 2017).

Apart of this change there are a few minor adjustments.

[markuspg]

In this pull request it's the "inline variables" from C++17 which I utilized. I thought some of the new containers would be practical too (e.g. to replace the union in the uuu_notify struct, although on a second look there don't seem to be much more opportunities to put them to use).

[nxpfrankli]

vs build failure