Fix install destination of headers

[marbre]

Without this fix the headers get, at least on my system, installed to /usr/include/compute/boost/compute instead of /usr/include/boost/compute.

[marbre]

BoostComputeConfig.cmake.in needs to be modified to if this patch is accepted. Before doing so, I would appreciate any feedback.

[kylelutz]

Yeah, this is the expected behavior. For now, you should install to /usr/include/compute and then pass -I/usr/include/compute to include the Boost.Compute headers. Once in the standard Boost release (should be soon now), Boost.Compute will be installed with the rest of the Boost libraries under /usr/include/boost.

[marbre]

Thanks for clarifying. Closing this pull request, as this is the expected behaviour.