Enable build of examples using system installed Boost.Compute

[ghisvail]

For c-i / packaging purposes, it would be desirable to allow building the examples (and perhaps the test suite) using a system install of Boost.Compute.

The necessary changes may be as simple as:

• Make example a separate CMake project.
• Add a call to FindPackage(Boost.Compute) at the beginning of the list file.
• Replace all occurrences of Boost.Compute by the variables set by FindPackage.