improve MPI makefile

[wds15]

Summary:

The MPI makefiles currently do not build boost in parallel and they also do not integrate well with cmdstan.

Description:

• Builds are slow due to not using parallelization of boost builds.
• make/setup_mpi adds to the global LDFLAFS and CXXFLAGS which is problematic for cmdstan.
• We should be able to use our standard CC=clang++ in make/local to configure the compiler used for the shared Boost libraries as well. By default it picks something installed on the system somewhat at random and if this is different than the compiler used to compile Stan programs we can achieve segfaults.

Reproducible Steps:

NA

Current Output:

NA

Expected Output:

NA

Additional Information:

Provide any additional information here.

Current Version:

develop

[SteveBronder]

@wds15 is this still an issue?