CMakeLists.txt changes to build as self-contained library

[PDBeal]

Modified CMakeLists.txt for the project to be built as a self contained library. This allows it to be included in the main project CMakeLists.txt without having to include the pico-sdk libraries or anything additional outside of just including the library.

[PDBeal]

Upon usage in an existing project, I found I also needed to change this section of CMakeLists.txt to add the framebuffer.cpp file

target_sources(${PROJECT_NAME} INTERFACE ${CMAKE_CURRENT_LIST_DIR}/ssd1306.cpp ${CMAKE_CURRENT_LIST_DIR}/framebuffer/framebuffer.cpp )

[Harbys]

Hey, i really appreciate your work! I should be able to test these changes in about a week