superenv: fix versioned g++ invocations.

Closes #24104.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
macvim 7.4-72

This release is essentially what was previously just the "head" version,
so remove all conditionals around head and leave only the head decision.

Closes #24108.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>