Allow build with distro libraries

[papoteur-mga]

Building dm-tree from source needs to download at build time pybind11. However, distros prohibit any download at build time, thus the build fails. The build does not use the library provided by the distro.
Probably abseil-cpp is in the same case.
Could the cmake configuration check if these libraries are already installed and uses them in that case?

[BwL1289]

Experiencing the same. Need to be able to use our system libraries. There's an open PR #73 for years to fix this.