Add Cmake install and find_package support

[offa]

Description of the change

Adds install and find_package support for CMake. Install commands (ie. make install) will now install all files needed.

Consumers can now use:

find_package(maddy REQUIRED)

add_executable(my_exe)
target_link_libraries(my_exe PRIVATE maddy::maddy)

(Readme is updated with that example)

Checklist for contributor

• if you want to be mentioned in the AUTHORS file, you added yourself
• added an entry to CHANGELOG.md at "Upcoming"
• if any Markdown definition changed, you updated the definition docs
• your C++ code change is accommodated with a unit/integration test where it makes sense
• your code meets the code format style (clang-format) of the project (tools/format.py)

[progsource]

Hi, thank you for your contribution. Feel free, to also add the find_package support. Would you want to do it in this PR or in another one?

[offa]

I have updated the PR 👍.

[progsource]

I consider writing a test build (like in tests/cmake) with a tiny example executable to check if the find_package works, but I can do that in an own PR later. (before releasing the next maddy version)
What do you think?

[offa]

In case you want to run the test locally too, you have to change the CMake prefix path, as the default shouldn't be writeable.

However, in most cases the install or cmake call should already issue an error if something is wrong.

[progsource]

Local test worked 🙂 looks good to me.

[progsource]

Thank you very much.