Work as an external project or add_subdirectory directory

[adamnovak]

This has the changes to the CMake build system I needed to make this project a well-behaved external project or subdirectory.

[adamnovak]

This should be enough to pass tests on Mac and Linux and to work as an add_subdirectory() in vgteam/libbdsg_example on Linux when installing the depending project.

I think to work on Mac with libbdsg_example when installing the depending project, we need to change the external_project() calls in this repo that result in dylibs into add_subdirectory() calls, so the actual final install step can be in charge of setting all the install names. But I think that will be another PR.

[adamnovak]

This should work as an external project too but whatever uses it that way would have the same install name problem on Mac that we have with our own dependencies.