Set set correct defaults (c++ 11) as a public property

[dblock]

What platform/OS are you using?

MacOS

Which version of the SDK?

Mojave

What compiler are you using? what version?

CMake 3.16.4

Coming from awsdocs/aws-doc-sdk-examples#1022 (comment)

Currently projects require setting the target compile features to C++ 11 in CMakeLists.txt with target_compile_features(my-example PUBLIC cxx_std_11) or risk getting the following errors:

[ 50%] Building CXX object CMakeFiles/list-s3-buckets.dir/main.cpp.o
In file included from /Users/dblock/source/aws-sdk/list-s3-buckets/main.cpp:2:
In file included from /usr/local/include/aws/core/Aws.h:17:
In file included from /usr/local/include/aws/core/utils/logging/LogLevel.h:20:
In file included from /usr/local/include/aws/core/utils/memory/stl/AWSString.h:20:
In file included from /usr/local/include/aws/core/utils/memory/stl/AWSAllocator.h:21:
/usr/local/include/aws/core/utils/memory/AWSMemory.h:162:78: error: 'T' does not refer to a value
            std::size_t headerSize = (std::max)(sizeof(std::size_t), alignof(T));
                                                                             ^
/usr/local/include/aws/core/utils/memory/AWSMemory.h:149:23: note: declared here
    template<typename T>
                      ^

There should be a way to not have to do this for every project.

[Lectem]

Note that it would require to have at least one exported target to have this compile feature requirement.

It should either be used (transitively) for all targets that get installed or it could be handled by the https://github.com/aws/aws-sdk-cpp/blob/master/cmake/AWSSDKConfig.cmake file.
Since it already iterates over targets it could probably call target_compile_features for each of them ?

It already tries to do something with a hardcoded CPP_STANDARD

aws-sdk-cpp/cmake/AWSSDKConfig.cmake

Line 71 in a80b3e7

set(CPP_STANDARD "11" CACHE STRING "Flag to upgrade the C++ standard used. The default is 11. The minimum is 11.")

that knows how to set the standard only for gcc/linux (see

aws-sdk-cpp/cmake/compiler_settings.cmake

Line 36 in bfec5f0

list(APPEND AWS_COMPILER_FLAGS "-fno-exceptions" "-std=c++${CPP_STANDARD}")

and

aws-sdk-cpp/cmake/platform/linux.cmake

Line 32 in 0912003

list(APPEND CMAKE_REQUIRED_FLAGS "-std=c++${CPP_STANDARD}")

)