The GHC 9.0.2 bindist expects the headers of libffi to be found under
/Applications/XCode.app instead of /Library (which is the case for later
GHC bindists which I copied the code from). Correctly substituting this
path will prevent GHC from using the bundled and thus outdated libffi
headers that are broken for some reason or other.
That these bundled headers were installed was actually a bug in hadrian
which created the bindists because it ignored use-system-libffi before
GHC 9.2. (This was actually the issue I noticed first because it hides
the actual problem: Because GHC installed these headers, GHC found some
version of ffi.h and ffitarget.h instead of correctly failing due to
missing headers which would indicate that package db is broken.) For the
sake of completeness, we can also solve this problem by deleting these
headers in postInstall.
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