"--enable-doc-gen" configure flag will enable HTML document generation that requires mdbook.
We can't simply disable it because we need manpages.

Author: Zhu Zihao <all_but_last@163.com>

--- a/doc/manual/local.mk
+++ b/doc/manual/local.mk
@@ -138,11 +138,6 @@
 	$(trace-gen) $(dummy-env) NIX_PATH=nix/corepkgs=corepkgs $(bindir)/nix __dump-builtins > $@.tmp
 	@mv $@.tmp $@
 
-# Generate the HTML manual.
-.PHONY: manual-html
-manual-html: $(docdir)/manual/index.html
-install: $(docdir)/manual/index.html
-
 # Generate 'nix' manpages.
 install: $(mandir)/man1/nix3-manpages
 man: doc/manual/generated/man1/nix3-manpages
@@ -167,23 +162,4 @@
 	done
 	@touch $@
 
-$(docdir)/manual/index.html: $(MANUAL_SRCS) $(d)/book.toml $(d)/anchors.jq $(d)/custom.css $(d)/src/SUMMARY.md $(d)/src/command-ref/new-cli $(d)/src/contributing/experimental-feature-descriptions.md $(d)/src/command-ref/conf-file.md $(d)/src/language/builtins.md
-	$(trace-gen) \
-		tmp="$$(mktemp -d)"; \
-		cp -r doc/manual "$$tmp"; \
-		find "$$t</td></tr></table>
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