Enable building for platforms other than Arduino.

[codygray]

With a few minor tweaks, this library can be made more universal, so that it can be built for platforms other than Arduino. This vastly increases the usefulness of the library, without compromising any functionality.

This merge request contains the changes needed to make the library universal enough to be built for any platform with a standards-compliant C++11 compiler. These changes are as minimally-invasive as possible, and remain fully backwards-compatible (as long as ARDUINO is #defined, the original code will be used).

[feliwir]

Would it maybe be possible to provide a millis function through a std:::function? On my STM32 chrono doesn’t work

[codygray]

Would it maybe be possible to provide a millis function through a std:::function? On my STM32 chrono doesn’t work

Sure, you could modify the header file to accept an implementation in the form of a caller-specified lambda. That probably makes more sense than attempting to detect the environment with preprocessor macros, since the way of doing this tends to vary widely across different microcontrollers and development environments.

But I would certainly prefer to use the standard implementation whenever possible for simplicity and elegance, so I'm not quite sure how to merge this in alongside what I have now. Perhaps checking for C++11 support with a macro? That would work, unless your compiler reports it is C++11-compliant, yet omits support for the std::chrono libraries. And without C++11 support, you're not going to have std::function or lambda support, either.

For compatibility with these older environments, it might make more sense to just link in your own millis() function. I don't know. Open to suggestions on this.

[mikalhart]

Plan to introduce a millis() function for non-Arduino platforms using std:chrono (where it is supported). Would be grateful if someone could do a PLATFORMIO test on the result.