add cmake exported targets

[aschier]

Add exported targets to make it easier to link against geometry-central.

Projects can then simply use

find_package(geometry-central)

to link against it and configure the location using geometry-central_DIR, which will also be automatically detected if geometry-central is installed in $CMAKE_PREFIX_PATH.
I also added the version (from the latest tag) to the CMakeLists.txt so that projects can require a specific version.

The targets should probably be handled by ${GC_DEP_LIBS}, but that causes a mismatch between the eigen and Eigen3::Eigen targets, and I do not want to change too much in your CMakeFiles.txt files. Possibly these dependencies could also be private in case that geometry-central links them statically.

[hollowsunhc]

Looks similar to what I'm achieving here #176

[aschier]

I'd happily use your commit when it gets merged. This one is the minimalist version to get it to run.
What do the maintainers think? There is little news on the other pull request as well.

[hollowsunhc]

Well I believe I understand. They need to check if it works because I'm barely providing any proof nor tests. Moreover, I've been using Windows to make the changes and test them. I cannot guarantee it works on Linux.

There could also be an issue related to naming and standardization. For example, should a given directory be called "geometrycentral", or "geometry-central"? find_package(geometrycentral) or find_package(geometry-central)? Ultimately, it's up to them to decide what they want. I'm just here providing some logic that worked for me 😄

EDIT: Also, it would be a breaking change, bumping the version to 1. They may not want that now.