CMake: Prefer relative paths instead of absolute paths
Since CMake 3.13 it is preferred to use relative paths to the current list directory rather than absolute paths. While we fixed most of these with !143 (merged), there are still some remnants, e.g.:
add_library(distributor STATIC)
set_property(TARGET distributor PROPERTY POSITION_INDEPENDENT_CODE ON)
target_sources(distributor
PUBLIC
${INCLUDE_DIR}/common/rpc/distributor.hpp
PRIVATE
${CMAKE_CURRENT_LIST_DIR}/rpc/distributor.cpp
)