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
I have no problem with that (patch locally). :-)

For the record, I build for arm-none-eabi on top of eCos embedded RTOS for legacy ARM SoCs (ARM7, ARM9).

About ARRAY_SIZE: again, this is quite a common macro, maybe not as common as MIN & MAX but it quite common to find it. We define it system-wide, the same way MIN & MAX are defined, but we also have 3rd party libs that redefine it.