Allow passing sdbus-cpp from system

[mweinelt]

Feature request

What problem does this feature solve?

As a distro (nixpkgs in this case) we want to unbundle vendored dependencies as much as possible to reduce the number of package versions we need to support in parallel.

What does the proposed API look like?

n/a

How should this be implemented in your opinion?

Create a USE_SYSTEM_CDBUSCPP_LIBS_LIBS cmake option.

Are you willing to work on this yourself?

Unfortunately my cmake skills are insufficient.