Missing "sal.h" when attempting to compile with MPI on Windows

[willend]

(From debugging-session with Steffen Sloth)

If compilation for MPI on Windows fails due to missing "sal.h", the solution is to (re-)install the MSYS mingw compiler present in conda: (from the current McCode/meta-pkgs/windows/)

conda install -c msys2 m2w64-gcc m2w64-gsl -y

(See also https://github.com/McStasMcXtrace/McCode/commit/728f5367d9d048c89bd1138e717335c6063d2172)