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A general description is here.
‘configure’ now adds a compiler switch for C++11 code, even if the compiler supports C++11 by default. (This ensures that ‘g++’ 6.x uses C++11 mode and not its default mode of C++14 with ‘GNU extensions’.)
The tests for C++11 compliance are now much more comprehensive. For gcc < 4.8, the tests from R 3.3.0 are used in order to maintain the same behaviour on Linux distributions with long-term support.