Add missing boost include directories

Unlike with other dependencies, there is no global include_directories() in the top-level CMakelists.txt file for Boost. Since this gives problems when building with Boost in a custom prefix, some target_include_directories() directives are added where needed.

Note that in one occasion PRIVATE is used and PUBLIC at the other. This is because the Boost header files are either included in a .cpp file, or in the .h file.