Makefile: Rework for use by haddock developers

Previously the Makefile was present only for GHC's old make-based build
system. Now since the make-based build system is gone we can use it for
more useful ends.

Makefile

@@ -1,15 +1,37 @@
-# -----------------------------------------------------------------------------
-#
-# (c) 2009 The University of Glasgow
-#
-# This file is part of the GHC build system.
-#
-# To understand how the build system works and how to modify it, see
-#      http://ghc.haskell.org/trac/ghc/wiki/Building/Architecture
-#      http://ghc.haskell.org/trac/ghc/wiki/Building/Modifying
-#
-# -----------------------------------------------------------------------------
-
-dir = utils/haddock
-TOP = ../..
-include $(TOP)/mk/sub-makefile.mk
+clean: ## Remove compilation artifacts
+	@cabal clean
+
+repl: ## Start a REPL
+	@cabal repl
+
+test: ## Run the test suite
+	@cabal test
+
+lint: ## Run the code linter (HLint)
+	@find driver haddock-api haddock-library haddock-test hoogle-test hypsrc-test latex-test \
+			-name "*.hs" | xargs -P $(PROCS) -I {} hlint --refactor-options="-i" --refactor {}
+
+style: ## Run the code styler (fourmolu and cabal-fmt)
+	@cabal-fmt -i **/*.cabal
+	@fourmolu -q --mode inplace driver haddock-api haddock-library
+
+style-check: ## Check the code's style (fourmolu and cabal-fmt)
+	@cabal-fmt -i **/*.cabal
+	@fourmolu -q --mode check driver haddock-api haddock-library
+
+style-quick: ## Run the code styler on modified files
+	@cabal-fmt -i **/*.cabal
+	@git diff origin --name-only driver haddock-api haddock-library | xargs -P $(PROCS) -I {} fourmolu -q -i {}
+
+tags: ## Generate ctags and etags for the source code (ghc-tags)
+	@ghc-tags -e -c
+
+help: ## Display this help message
+	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | awk 'BEGIN {FS = ":.* ?## "}; {printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}'
+
+PROCS := $(shell nproc)
+
+.PHONY: all $(MAKECMDGOALS)
+
+.DEFAULT_GOAL := help
+