Improve CMakeLists a little

[redstrate]

Hello! I use this project as a git submodule (and add it via add_subdirectory) and it doesn't quite work as I expected. Making it work is super simple though:

• Added the target_include_directories function so it's usable if you link it via target_link_libraries(myapp PRIVATE cotp)
• Used the target_X versions of the functions like link_options, so it doesn't pollute everything around it. This might have broken tests though, I need to check.
• Removes the explicit SHARED so it listens to BUILD_SHARED_LIBS which is a standard CMake variable. It's also added as an explicit option, and turned on by default. It's still possible to override this, which I do.
• Adds an option for building tests, but it's on by default.

[paolostivanin]

closing as there was no further feedback