Provide useful warning message about missing `compile_commands.json`

[kammce]

Currently if the compile_commands.json doesn't exist, CMake produces this output. It would be nice to have a better message using the cmake "Warning" logging type.

Error copying file (if different) from "/Users/kammce/git/libhal/libhal/v4/build/Debug/compile_commands.json" to "/Users/kammce/git/libhal/libhal/v4/compile_commands.json".