Optionally use installed sdbus-cpp library

[jnnks]

This PR implements a compile-once solution for the external project sdbus-cpp used for the docker container build toolchain.
The installed library is used instead of building the in-tree module when it can be found with cmake find_package.
A fallback was implemented to avoid build errors in case the library is not installed on the host.

This is a small study to reduce build times by providing precompiled resources before Anbox is built.
The performance improvement is for niche use cases where the entire project is fully rebuild multiple times.

By introducing other concepts, the overall build time is reduced anyways. For example docker BuildKit, which reduces the buld time for the anbox-build image.

job	master (s)	this PR (s)	diff (master - PR in s)
scripts/build.sh	60.41	59.51	0.9
scripts/build-with-docker.sh	61.28	60.15	1.13
scripts/build-with-docker.sh (docker --no-cache)	163.44	156.99	6.45

When accepted this will be a reference implementation with the other external modules following in the same style.

@morphis In src/CMakeLists.txt the include path for sdbus-cpp is hardcoded. I would like to use something like sdbus-c++_INLCUDE_DIRS, but the variable was empty. Do you know a work-around?

[Niek]

Alpine is using a similar patch: https://gitlab.alpinelinux.org/alpine/aports/-/blob/master/community/anbox/no-bundled-sdbus.patch

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[Fuseteam]

What is this missing?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[Fuseteam]

Still sounds useful...