Simplify CMake

[lederernc]

Simplify the CMakefile.txt to make it easier to maintain. Remove visual studio solution files as they are not necessary and hard to maintain properly.

I also converted the library to build as a static library. This is because the DLL being generated on windows was not exporting symbols properly. I can add proper support for shared vs static building in a later pull request.

I added compiler flags to Visual Studio builds to match the warnings as errors flags on non-VS builds.

[AngusJohnson]

Thanks Nicolas.
I've merged your changes which look just fine.
Howevr, I'm a complete novice with CMake so there's an element of trust there 😁😱🤞🙏.
And on that note, how do I get CMake to generate solution files so that Visual Studio will compile and execute the examples.
I'm sure there are online examples re using CMake in VS but I'm hoping you can provide a "quick and dirty" explanation that's specific to your modified CMakeLists.txt. I've gotten as far as

option(CLIPPER2_UTILS "Build utilities" ON)
option(CLIPPER2_EXAMPLES "Build examples" ON)
option(CLIPPER2_TESTS "Build tests" ON)

but no further. Cheers.

ps: I'm very likely going to need a reasonably thorough explanation, at least one that I can add to the documentation.
(I suspect there will be many Visual Studion users that are equally ignorant WRT CMake files.)