cmake: add cache variables for BUILD_SHARED_LIBS & VVENC_LIBRARY_ONLY

[K-os]

closes #433

[adamjw24]

What happens to make test if you do that?

[K-os]

Make test will fail, naturally if you set VVENC_LIBRARY_ONLY=1, but I guess that is how it is. How should you run the tests, when you don't build the tests?
You can still go to your build directory (e.g. build/release-static), explicitly build all the binaries, and then run ctest.

But why would you set VVENC_LIBRARY_ONLY in the first place, if you want to run the test suite?

[K-os]

We do output a warning now that the test suite will not be able to run, when enabling VVENC_LIBRARY_ONLY.