Ensuring that there is no 'main()' function in user's code when compiling for Arduino

[divanchykhin]

Arduino provides its own staff including their own 'main()' function. If user's code happens to have a 'main()' function, too, it can be silently used by a compiler instead of one from Arduino thus producing undesired result. As a potential solution it might be good, in case of Arduino (or, more generally, in case of any framework providing its own code) to check whether user-defined code has a 'main()' function and to report an error.