Draft: Replace findSharedMemory() with use of shmem interface allowing mbuf/posix implementation switching.

Adding mbuf include path because shmem.c includes mbuf.h and I did not want to change that, adding lrt for posix shmem linkage, adding explicit command to link the executable with LDFLAGS (we were using make default), changing $< to $^ so all prerequisites are used and not just the first which works in the single prerequisite case only, adding build and link with shmem.c, adding target for the MODELNAME.o instead of relying on the default command make was using.