cmake interface library

[plusk01]

This PR "modernizes" the cmake build process:

1. marks the plot header files as interface includes so that cmake targets using target_link_libraries automatically pull in the headers (used here)
2. optionally disables building the applications -- useful when plot is included in a superbuild (used here)

Thanks for the cool plotting library. Would love to see more features be added (e.g., axes, legends, etc.). FYI, I'm using this library here (with eigen).

[fbbdev]

Hi,

thank you for the PR. As you can see from open issues, proper plotting features (axes, scaling, etc.) have been on the roadmap since long ago.

The reason I stopped at the time was that I didn't yet have a clear idea about how to design a clean, usable API.

Unfortunately I just don't have any time to work on this, but proposals, discussions under #15 and #16 and eventually PRs are very welcome.