I do not think we need a safeguard against not including files in
CMake anymore, and as it is, it caused annoying false positive about
the default main implementation.
This describes the reality better, as it also links in the rest
of Catch2.
The on-disk name of the static library remains just `Catch2Main`,
as that is what it is -- single main function -- and on-disk artifacts
cannot describe link dependencies.
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
* Every Linux build tracks coverage when running Debug mode
* OS X not supported yet (Future WIP)
* Our own unit tests, non-default reporters and Clara are ignored
