Makefile: clean up Makefile

Makefile: clean up Makefile

There are a number of inconsistencies and superfluous arguments:

- For libmymemory, we don't need (or want) to hardcode a shared library
  relative path
- For libmodel, we don't really need the 'soname' parameter
- Linking C++ probably should be done with the C++ compiler, not the C compiler
- We don't need CPPFLAGS for linking-only stages
- We want to use LDFLAGS for an early-stage linking rather than the late-stage
  linking of the user program

Overall, simpler is better