Restructure the project

It was failing on 64-bit platforms due to insufficient configuration
in mlink and a51: DWORD was mapped to 64-bit long.

Added the necessary configuration and restructured the project:

- shared include files go under include/
- shared portability code is under lib/port and is compiled into a
  library
- rename src/ to a51/
examples/Makefile: a makefile to build the examples