Compile LMMS using C++17

Browse Source

This replaces `set(CMAKE_CXX_STANDARD 14)` by `set(CMAKE_CXX_STANDARD 17)`
wherever it is required.

Additionally:

* raise `CMAKE_MINIMUM_REQUIRED(VERSION ...)` to `3.8` (the minimum
  that supports C++17)
* `stdshims.h` is now unused and thus removed

...

This commit is contained in:

500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f