Because of a change in VS toolset, missing option <UseFullPaths>
is no longer interpreted as "don't pass /FC to the compiler", but
rather as "pass /FC to the compiler". This is problematic, because
/FC not only changes how much of the path is reporter by the compiler
(e.g. in `__FILE__` macro), but it also lower cases the path.
This lower-casing of the path broke our approval tests for VS2017
about 5 months ago.
Using CMake 3.13 (not yet released) would also let us fix it, but
for now we use a vcxproj.user file that is merged with the main project
and explicitly disables `/FC`.
The support is to be considered experimental, that is, the interfaces,
the first party generators and helper functions can change or be removed
at any point in time.
Related to #850
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
