Modified Arduino.h with changes to the boolean type

As the Arduino specific boolean type is dangerous in C++, I have implemented a fix to solve the situation for both C and C++ There is also no need for stdbool, it also defines true and false, which is not needed or allowed in a C++ context.

This is in reference to the issue I posted here.

https://github.com/arduino/Arduino/issues/2147
This is the important part of my addition. The boolean type needs to be seen as a bool when compiling C++ code.