Multiple changes necessary to enable this. The main goal is to build
and run the C++ ioinit unit tests (which require c++11)
* Updated mraa required CMake version to 2.8.11 (this is needed for
the target_xxx_xxx CMake syntax.
* Added function for adding the c++11 flag give a CMake target.
* Updated unit tests for range of CMake versions (tested on 2.8.11,
3.8.2, 3.9.6, and 3.12.0).
* Added C++ unit test file (more needed here).)
Signed-off-by: Noel Eck <noel.eck@intel.com>
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