5.1: source-tree timestamps still matter

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I've been building 5.1 from source a lot over the last few days, and I
just did a build after copying  source tre in a non-mtime-preserving
way, and the build crashed in a way that touching a source file fixed.
(Each build - before and after the touch - was done with -D and -O
naming completely empty directories, so it's not leftover bits from a
previous build causing trouble.)

Thus, I infer that 5.1 is not completely free of "source file
timestamps matter" issues.  Under 4.0.1, IIRC, people cared about
these; do people still care about them under 5.1?  If so, what should I
do?  Just file (toolchain?) PRs?

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