CMake: use cxx_std_11 compile feature instead of CMAKE_CXX_STANDARD

[SpaceIm]

• cxx_std_11 allows to override C++ standard externally while CMAKE_CXX_STANDARD hardcodes it internally, preventing any change for consumers to set another standard like C++14, 17, 20 etc (usually users like to use the same C++ standard in the all dependency graph).
• cxx_std_11 is propagated to features of import target in CMake config file

[berndporr]

This moves it up a lot in the cmake version number. However, there are people who use Linux distros with cmake 3.1.0 and as long as we are using C++11 (which is 10 years old!) I'd keep it at 3.1. It's actively used and would break dependency.

[SpaceIm]

As you wish. You may be stuck to 3.1 for years if you want to support default CMake version of old distros (by the way users can update their CMake version, even on old distros).

[nickbailey]

A lot of our users don't have root privileges, so it's difficult for them to update the current cmake. The develop software as users. They would have to build a local version, which is more than trivial. I have to agree with @berndporr on this one, although normally newer is better, obviously.

[berndporr]

I've found this post here: https://unix.stackexchange.com/questions/535052/what-cmake-version-can-i-rely-on-in-current-popular-linux-distributions Looks like 3.8.0 seems to be OK so I've committed the changes.