72d0f85dd2
zeroc-ice used to smuggle a forked mcpp version in with an override,
which broke after applying a security patch against mcpp in
c60cafa719. Overriding instead of defining
a new derivation is a questionable optimization since in fact, only the
'configureFlags' line is shared.
Remove the override and give the forked mcpp a live on its own.
The security patch for mcpp is not relevant for this fork.
Fix #98581