Consider making Boost.Compute non-header-only

Consider making Boost.Compute non-header-only. This issue was raised a few times during the Boost formal review for Boost.Compute. There are a number of potential advantages including allowing the library to be thread-safe by default, reducing the amount of code in header files to improve compile times, and allowing the build system to convert `.cl` source files to strings in `.cpp` source files.

This would be a large and non-backwards-compatible change and should be discussed with the community. Downsides include making the library slightly more difficult to use as it would now require separate compilation and installation steps before use.
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