cpp headers not in target_include_directories

[DanielWe2]

The cpp headers are not added to the target_include_directories. Without that it is hard to use CRoaring from a local subdirectory from C++ without install. (it can not find the cpp headers)

One would probably need to add a new subdirectory "roaring" into the cpp directory to do that in a clean way.

[lemire]

Sounds reasonable. PR invited.