fixup_mmaps_after_fork errors building ruby 3.4.1 with high make -j

[Jeremy Drake]

On Tue, 21 Jan 2025, Jeremy Drake via Cygwin wrote:

> I did not see this happen with ruby 3.3.7, or when building with -j1.

Sorry, spoke too soon, I did see this immediately with 3.3.7 as well.