Ability to customize the library's C++ namespace (default is lbstd)

[leha-bot]

via CMake's configure argument -DLBSTD_VERSION_NS=<custom_namespace>
The macro for name of namespace will be LB_STD_VERSION_NS (with underscore, inconsistent, yes).
Another tasks:

• Passthrough the CMake arg to C++ #define;
• Test in tests/CMakeLists.txt for checking the baked C++ namespace's name with baked into CMake;