Need to update to align with the approach taken in `mpif/1.0.0`

[ocaisa]

See https://github.com/eschnett/mpif/blob/main/ci-scripts/install-mpi-stubs.sh

Will require a bit of a redesign, separating out the compilation of the original and mpif parts for mpi-abi-stubs. Do-able but needs a bit of thinking.

[ocaisa]

Mostly done, just need to tweak how mpi_proxy.c is created a bit, but what is there works now (and is proven via CI with the tests from mpif)