ViennaCL: Added hybrid capabilities (CUDA, OpenCL, OpenMP)

Depending on whether the user configures with CUDA, OpenCL, or OpenMP,
the respective backends will be available in ViennaCL.
CUDA has prec

ViennaCL: Added hybrid capabilities (CUDA, OpenCL, OpenMP)

Depending on whether the user configures with CUDA, OpenCL, or OpenMP,
the respective backends will be available in ViennaCL.
CUDA has precedence over OpenCL, which has precedence over OpenMP.
If none of the backends is selected, ViennaCL is built in sequential mode.

Remaining tasks to better leverage these hybrid capabilities:
- Select backend at runtime (API + options)
- Eliminate copy-overhead for OpenMP backend
- Documentation, examples, and tests to demonstrate usage

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