Fix small compilation issues

* Add missing “assert.h” file header inclusion
* Do not define ARRAY_SIZE if it is already defined
* “mpconfigport.h” is a library header, not a system header
Yes, sorry, I used the bad terms, "local" is far more appropriate. Ok, not a big deal anyway, it is easy to change it on each installation. The way I build it is "local" - or should I say project-wide, so < > fails to build properly, but again, it is quite easy to change.