Refactor examples' cmakes

[lukaszstolarczuk]

Rationale

We want to test all examples in the CI and make sure CMake file for each example is proper.

Description

Make use of "per example CMake" (e.g. examples/pmemkv_basic_c/CMakeLists.txt) in the main example CMake (examples/CMakeLists.txt). It will reduce code repetition and actually tests if all our examples work.
We could also make sure all our examples are tested in the CI.

API Changes

no

Implementation details

Use include function or somehow make sure to enter all examples' sub-directories and compile every single example.

[lukaszstolarczuk]

partially done - we assured all examples are executed in CI (in #1045)