How to create your own `consts.jl` file manually

[sloede]

As a tangent to #574, I am wondering how to go about when trying to create your own constants file manually, i.e., one that could live along the others in src/consts? Would it be ok to just copy one file, e.g., the one for OpenMPI and look at my own MPI implementations mpi.h and change everything that is different? Or how would I go about this properly?

Also, regarding proprietary MPI implementations: Does anyone know if this potentially touches copyright issues if I create such a file and provide it publicly, or is it kosher because it only describes the API/ABI and nothing else?