Modernize cmake

[mattangus]

This PR moves to a more modern target based CMake usage. This avoids setting properties globally for potential consumers of this project.

[stribor14]

Hi Matt, thank you for the contribution. I generally agree with the change as a correct direction, but for now I would would like the merge to point to devel-v040 branch as for us this change would mean "breaking API change" (on some of our ROS1 "catkin" deployments we would have to write additional CMakeLists wrapper just to re-add flags again).

Another option would be to "hide" these compile options behind a custom option which could be easily disabled/overridden when including this file in other projects. (e.g. EIGENCPP_OPTFLAGS and EIGEN_FLAGS in OCaml wrapper for eigen for Owl)

[stribor14]

Additionally, we planned to revisit default compile flags for v0.4.0 release at #48 (e.g., -DNDEBUG, -O3 instead of -O2, etc.), but this will be done at the end after it is feature complete (there are some API changes + new functionalities are still in testing, especially curve offsets and polyline fitting)

[mattangus]

Closing in favor of: #53