Adds basic support for cmake.

[atuleu]

Knowning exactly what to do to use can2040 in a project can be a bit hairy.

This provide a CMakeLists file that is compatible with the pico C/C++ sdk to build the library. It is compatible with FetchContent.

Therefore including can2040 in a project can be done with

pico_sdk_init()

FetchContent_Declare(can2040
GIT_REPOSITORY https://github.com/KevinOConnor/can2040.git
GIT_TAG master)
FetchContent_MakeAvailable(can2040)
add_executable(myprog)
target_link_libraries(myprog can2040::can2040)

[KevinOConnor]

Thanks for working on this. In general, I agree it would be useful to have a build example in the can2040 repo. I have a few comments though:

1. I'd prefer to have a single directory containing all the files needed for the example (eg, example/) and not have any files other than that in the root of the repo (that is, no .gitignore changes, no cmake files).
2. I have not documented this, but I think it would be worthwhile to follow the Linux kernel style developer certificate of origin and have "signed-off-by" lines in every commit ( https://www.kernel.org/doc/html/latest/process/submitting-patches.html?highlight=signed%20off#sign-your-work-the-developer-s-certificate-of-origin ). I can update the documentation to reflect this requirement. It would mean contributions to can2040 would require agreeing to the developer certificate of origin and providing a full real name.
3. The example code should have consistent indentation.

Cheers,
-Kevin