Remove an option in Openmpi build which broke compilation on case insensitive macOS
There was an issue when Openmpi was compiled on macOS with case-insensitive filesystem: some includes in the libc++ STL were incorrectly including the VERSION file from Openmpi, resulting in compilation error... (see this Openmpi issue I wrote for more details).
Someone pointed out it was likely due to the fact I selected the option to enable mpicxx, which is deprecated for quite some time now. I don't remember why I added it (I never used the interface but remember at some point I had to add it) but it is better anyway to remove it: it fixes the aforementioned problem, and it will anyway vanish in upcoming Openmpi 5.