CMake: set required C++ standard via target_compile_features()

[lyorig]

Currently, rectpack2D/CMakeLists.txt doesn't really communicate what C++ standard version it requires, there's only a set_properties() call in example/CMakeLists.txt that applies to the example executable itself. This is a minor edit that follows the build system's "best practices" and applies a required version to rectpack2D::rectpack2D that CMake (and perhaps other build systems like Meson) can better understand and work with.

[lyorig]

Now that I see "initial" as the most recent commit in both the src and example directory, I should've probably picked a more descriptive name for the commit... sorry about that.