There are some examples on issue #850 of using this feature, but they
are not easily found from the documentation. Adding them here as an
example makes them more findable and ensures they keep working if the
API changes.
Prevent warnings
- gnu: -Wcomment: multi-line comment
- clang: -Wweak-vtables 'class' has no out-of-line virtual method definitions; its vtable will be emitted in every translation unit
- clang: -Winconsistent-missing-override: 'method' overrides a member function but is not marked 'override'
- MSVC: C4702: unreachable code
This also goes for pkg-config installed by our CMake installation.
This includes
* Updating CMake version on Travis
* Adding a `Catch2` subfolder to the `single_include/` folder to
provide this include path both _inside_ the repository, and _outside_.
* Updated examples to build with the new paths
* Other general CMake cleanup
