modernized CMake

[jeffamstutz]

This PR adds a nanortConfig.cmake and then uses it in the local examples which are built with CMake. The config works like any other CMake package config, where

find_package(nanort)

...generates the following targets:

• nanort::core --> use only the "base" version of nanort
• nanort::threads --> use nanort with C++11 threads enabled
• nanort::openmp --> use nanort with OpenMP enabled
• nanort::nanort --> a target which selects OpenMP/threads/core (in that order) depending on what is available

The config requires CMake >= 3.9 to get the target-based version of CMake's find_package(OpenMP) support.

I ended up getting slightly carried away with further modernizations to the examples, which are stand alone commits to adding the config.

Let me know if this looks helpful/good and if you would like to see any revisions.

[syoyo]

@jeffamstutz Awesome! :tada:

Let us give some time review the PR.

[jeffamstutz]

I have no idea if this is helpful or not, but I recently just posted a vlog on CMake configs where I ended up using the config I wrote in this PR as content to talk to. So if you literally want me to walk you through what's there, here you go! https://youtu.be/GmcMct7LJuE?t=517

The existence of it in that video doesn't at all prevent any changes you might request. ;)

[syoyo]

Merged! Thanks!

TODO: Add usage doc to README.md