modernize CMake for submodule

[geoffviola]

This should be friendlier for people who expect the project to work as a git submodule. With modern CMake, all the include directories come via target_link_libraries

[caselitz]

this issue might be of interest regarding this pr

[geoffviola]

Yes, that looks related. That issue seems related to installation. This changeset is focused on using add_subdirectory(g2o) instead of find_package(g2o). I expect that both methods work in the modern way.

It looks like this changeset is currently incomplete because it is failing on the CI. I'll take a look later this week.

[sjulier]

Haven't had time to chase up on this for a while (teaching got in the way!) However, last time I checked it (January) everything worked okay on both macOS and Linux (although Ubuntu 16.04 ships with a surprisingly old version of eigen) but it needed more testing. Also I found very little documentaton on the "right way" to do it in cmake, and so I was hoping to check it's actually done right.

Unfortunately, I've got project review now and won't really be able to look at it for a few weeks. However if somebody else wanted to take ownership...

[geoffviola]

@sjulier The right way to do CMake is evolving. There are some good talks. I've been trying to push ideas from https://docs.google.com/presentation/d/1LvpDU25sr7AF8RLArSrY3KpZLV2qfnml3XuN5MpeCkw/edit#slide=id.g2f12105e36_0_89.

I'm afraid I don't rely on this library enough to want to be a dedicated owner. I'm just trying to reduce pain points in and out of my organization.

I'm still experimenting with new versions of Eigen. There's some insanely tricky error at runtime with -march=native, but I haven't debugged it yet.