reducecpp sample not passing correct -cl-std version

The "reducecpp" sample tries to build an OpenCL kernel that uses the work-group reduction functions, but does not pass a `-cl-std` version for this kernel. Since these functions require OpenCL C 2.0 or OpenCL C 3.1 or newer with the `__opencl_c_work_group_collective_functions` feature, some compilers will diagnose an error for this kernel.

This sample should pass a proper `-cl-std` build option for this kernel instead, similar to what is done for the non-C++ "reduce" sample.

Start with the reducecpp sample and compare how the non-C++ reduce sample passes its OpenCL build options. Identify where the kernel is built, add the appropriate -cl-std option there, and verify that the reducecpp sample builds successfully with compilers that require an explicit OpenCL C version.

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