Use CMake targets for linking against Boost

[DavidZemon]

I was running into a problem with absolute paths in the CMake scripts generated by freeopcua when a package is built. The scripts executed by find_package(freeocpua) contained absolute paths to the Boost libraries, which then failed if the package was sent to a different machine where boost was installed somewhere else. By switching the target_link_libraries invocations to use CMake targets instead of macros, this problem disappears.

[DavidZemon]

Hello maintainers. Is there any way you could merge this in? I'd really like to switch back to using your upstream source rather than my forked version, but cannot until this is merged.