How can I access directories outside of project root?

[gittemup]

I have a c++ project and want to use buck2 as my build system. The project includes c++ files outside my project root. How would buck2 handle including files outside project root?
Given an example like this: where /usr/share/file*.cpp and project root is /home/username/project_root/BUCK
eg. cxx_library( name = "cpp_project", srcs = ["/usr/share/file1.cpp", /usr/share/file2.cpp"], visibility = ["PUBLIC"], )

[lf-]

This is forbidden AFAIK. You might want an external cell for said out-of-tree code.