Haddock's Sphinx documentation breaks `make install` if doc building is disabled

slyfox reported that make install fails on his setup which lacks sphinx,

/usr/bin/install -c -m 644  libraries/prologue.txt "/var/tmp/portage/dev-lang/ghc-9999/image/usr/share/doc/ghc-9999/html/libraries/"
/usr/bin/install -c -m 755  libraries/gen_contents_index "/var/tmp/portage/dev-lang/ghc-9999/image/usr/share/doc/ghc-9999/html/libraries/"
for i in utils/haddock/doc/haddock; do \
        cp -Rp $i "/var/tmp/portage/dev-lang/ghc-9999/image/usr/share/doc/ghc-9999/html"; \
done
cp: cannot stat 'utils/haddock/doc/haddock': No such file or directory
ghc.mk:936: recipe for target 'install_docs' failed
make[1]: *** [install_docs] Error 1
Makefile:129: recipe for target 'install' failed

Trac metadata

Trac field	Value
Version	8.0.1-rc3
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
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Operating system
Architecture