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
Well, you don't say what and how you build, so we may only guess (and then we may guess that you do something wrong, because it all works for us). It would help if pull requests contained not just short list of changes, but description why they are needed (either as part of commit message or as a separate comment, whatever is more appropriate). For example, I'm not sure that ARRAY_SIZE change is "good" either.