Link warning from icpc due to mpicc/mpicxx mismatch

[Paul Hargrove]

When using the Intel compliers and MPI at JLSE to look into GASNet-EX bug 4039 , I was seeing the following from a upcxx -g ... invocation which include linking:

icpc: command line warning #10148: option '-Wunused-result' not supported

This turned out to be "pilot error".
GASNet-EX probed a gcc-based mpicc as it linker, and then an intel-based mpiicpc was used as CXX.

I believe we are lacking any documentation that mentions a need to specify an mpicc when seting CXX to an MPI C++ compiler. This is particularly problematic with the Intel MPI, where mpigcc and mpicxx co-exist as means to select the compiler family, and mpicc picks one via env var.

[Dan Bonachea]

This was discussed in the 2020-02-12 meeting

Paul will attempt as time allows

[Paul Hargrove]

proposed resolution appears in pull request 170

[Paul Hargrove]

docs: mention that mpicxx/mpicc/mpirun must match

This commit resolves issue 308.

→ <<cset be5c8fc3c0a0>>