set default c11 compiler for apple machines

[carstendroesser]

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Tell cmake which compiler to use on apple machines: Building the project out of the box on apple machines will cause Cmake to fail. Setting the correct default compiler will fix this issue. Extending CMakeLists.txt fixes the build-process and enables an 'out of the box' build without errors.

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@pmai

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[pmai]

I think whether C++ 11 is needed or not depends more on the protobuf implementation you will use (i.e. protobuf <3.6 does not need it, protobuf >3.6 needs it), than it does on OSI itself.

That being said, since 3.6 has been out now for more than 2.5 years, and C++11 for nearing 10, it might be a good idea to switch to C++11 by default (people who want/need to support pre-C++11 can just override the option, and supply suitable protobuf versions). The OSMP examples already require C++ 11.

However if we do so, we should switch across all platforms (there is nothing Apple-specific about this): Could you refactor the PR to just do this unconditionally? Thank you.

[pmai]

Looking good to me, ready for CCB discussion...

[kmeids]

Output CCB meeting 03.02.2021
Actions:

1. @pmai to merge.