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We used to call the bootstrapping compiler to get its --supported-languages
but that doesn't work when it doesn't support all the extensions that we
need. In particular, compiling the new bytestring which needs NamedFieldPuns
broken when building with 6.8.2.
There's now a rather ugly sed script to extract the extensions from
DynFlags.hs.

To quiet those warnings from make about not being able to delete
directories.

Since we started using "| $$$$(dir $$$$@)/." they've started causing
problems for unknown reasons.

rather than our own copies

This means that PACKAGE_TARNAME gets set, so the default docdir is
correct.

There are now 104 calls to mkdirhier, down from 1201, when validating.

This one complains sometimes, but there's no good way to improve it.

Tidy up the way that predicates are handled inside types

on Windows, as they are now in-tree

This helps on a hyperthreaded CPU by yielding to the other thread in a
spinlock loop.

The changes I made to the tidier recently introduced a serious
regression: the unfoldings for bindings in a recursive group were
sometimes lost, because we were looking at Id occurrences rather than
Id binders, and the occurrences of recursive Ids do not necessarily
have unfoldings attached.

Also add support for testing the bindistprep tarball, for when we are
validating.

This should catch make going into an infinite loop.