Hardcoded Paths in Makefiles

[xcelina]

At least some of the Makefiles in this project hardcode paths to MPI, NCCL, etc. at the lowest directory level, which means a user from a different system has to change those defaults just to be able to build the project. Example: microbenchmark/scale_out_cl/src/Makefile

There are shared.mk files to centralize that and they are used in SOME of the Makefiles, but not in most of them. Please fix that.