Make sure that spice can be used by another project via add_subdirectory()

[denniskb]

This requires the following steps:

1. Create a CMake-based C++ project
2. Add Spice as a sub folder, either manually by copy-pasting it (after obtaining it thru cloning or downloading the zipped version of this repository) or by adding it as a git submodule.
3. Inside the CMakeLists.txt, use add_subdirectory() to add Spice and link against it using target_link_libraries().
4. To make sure everything works, you should copy-paste one of Spice's samples into your own application and run it.