#1 API/ABI should not depend on compile-time defines

[Terence Simpson]

The API (and ABI) of the library varies depending on if DBUSPP_WITH_THREAD is defined, this makes a library compiled with this defined (or undefined) both API and ABI incompatible with applications/libraries compiled with a different option.
Instead, I would suggest having all members/methods declared in the header and the implementation depend on the compile-time value of such defines, this way an application would be able to compile against/link to the library whatever the value.