Libraries auto-detection update

Hi guys!

This pull request fixes the slow compilation from issue #280 and is up to date with `master`.

I got it fixed by using `:=` instead of `=` for [`USER_LIBS`](https://github.com/WeAreLeka/Arduino-Makefile/commit/58a2ebd367f691291bb268a854d2547285c4366b), [`MAIN_LIBS` and `MAIN_LIBS_DEPS`](https://github.com/WeAreLeka/Arduino-Makefile/commit/8305fa662f03c86e5e5cddfcca8309f4463526d5).

It is open for testing and should be good for everyone. :)