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and add a comment to explain why it was wrong.  This fixes the dyn
test failures that sprang up recently.

when we think that the libffi build creates them, so they just depend
on the libffi build stamp.

This avoids unnecessary work and potential loss of information

We do this through a gnu as feature called subsections,
where you can put data/code into a numbered subsection
and those subsections will be joined together in descending
order by gas at compile time.

The default object mode is already HscC, so it's unnecessary, and
-fvia-C generates a deprecated flag warning now.

This may not be ideal, but it should get GHC building on all platforms
again.

see comment for details

This fixes a problem with commands like gzip, where if $GZIP is exported
in the environment, then when make runs a command it'll put the Makefile
variable's value in the environment. But gzip treats $GZIP as arguments
for itself, so when we run gzip it thinks we're giving it "gzip" as an
argument.

This lets automatic prefetching work better, for a tiny performance boost

Get the set of free variables from the generated case expression:
includes variables in the guards and decls that were missed before,
and is also a bit simpler.