Avoid unconditionally setting no optimization and debug symbols

[danra]

in macOS/Linux release builds

[Arcnor]

Hi, thanks for your PR!

LIST(APPEND CMAKE_CXX_FLAGS... cannot really work, as CMAKE_CXX_FLAGS is not a list (it's a space separated string, you can see at https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_FLAGS.html)

You can do instead STRING(APPEND... which will use spaces instead. Other than that, I think it looks good.

[danra]

Good catch, will fix.
Also #46

[Arcnor]

Superseded by #54