Enhance `Makefile`

* `install` recipe does not rebuild binary
* Add `help` target and documentation
* Add several config variables
* Add sources as dependencies for (in-)validation

Based on template
https://gist.github.com/straight-shoota/275685fcb8187062208c0871318c4a23

Makefile

@@ -1,34 +1,92 @@
+.POSIX:
+all:
+
+# Recipes
+
+## Build ameba
+##   $ make
+## Run tests
+##   $ make test
+## Install ameba
+##   $ sudo make install
+
+-include Makefile.local # for optional local options
+
+BUILD_TARGET ::= bin/ameba
+
+DESTDIR ?=          ## Install destination dir
+PREFIX ?= /usr/local## Install path prefix
+BINDIR ?= $(DESTDIR)$(PREFIX)/bin
+
+# The crystal command to use
 CRYSTAL_BIN ?= crystal
+# The shards command to use
 SHARDS_BIN ?= shards
-PREFIX ?= /usr/local
+# The install command to use
+INSTALL_BIN ?= /usr/bin/install
+
 SHARD_BIN ?= ../../bin
 CRFLAGS ?= -Dpreview_mt
 
+SRC_SOURCES ::= $(shell find src -name '*.cr' 2>/dev/null)
+DOC_SOURCE   ::= src/**
+
+.PHONY: all
+all: build
+
 .PHONY: build
-build:
+build: ## Build the application binary
+build: $(BUILD_TARGET)
+
+$(BUILD_TARGET): $(SRC_SOURCES)
 	$(SHARDS_BIN) build $(CRFLAGS)
 
+docs: ## Generate API docs
+docs: $(SRC_SOURCES)
+	$(CRYSTAL_BIN) docs -o docs $(DOC_SOURCE)
+
 .PHONY: lint
-lint: build
-	./bin/ameba
+lint: ## Run ameba on ameba's code base
+lint: $(BUILD_TARGET)
+	$(BUILD_TARGET)
 
 .PHONY: spec
+spec: ## Run the spec suite
 spec:
 	$(CRYSTAL_BIN) spec
 
 .PHONY: clean
+clean: ## Remove application binary
 clean:
-	rm -f ./bin/ameba ./bin/ameba.dwarf
+	@rm -f "$(BUILD_TARGET)" "$(BUILD_TARGET).dwarf"
 
 .PHONY: install
-install: build
-	mkdir -p $(PREFIX)/bin
-	cp ./bin/ameba $(PREFIX)/bin
+install: ## Install application binary into $DESTDIR
+install: $(BUILD_TARGET)
+	$(INSTALL_BIN) -m 0755 "$(BUILD_TARGET)" "$(BINDIR)/ameba"
 
 .PHONY: bin
 bin: build
 	mkdir -p $(SHARD_BIN)
-	cp ./bin/ameba $(SHARD_BIN)
+	cp $(BUILD_TARGET) $(SHARD_BIN)
 
 .PHONY: test
+test: ## Run the spec suite and linter
 test: spec lint
+
+.PHONY: help
+help: ## Show this help
+	@echo
+	@printf '\033[34mtargets:\033[0m\n'
+	@grep -hE '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) |\
+		sort |\
+		awk 'BEGIN {FS = ":.*?## "}; {printf "  \033[36m%-15s\033[0m %s\n", $$1, $$2}'
+	@echo
+	@printf '\033[34moptional variables:\033[0m\n'
+	@grep -hE '^[a-zA-Z_-]+ \?=.*?## .*$$' $(MAKEFILE_LIST) |\
+		sort |\
+		awk 'BEGIN {FS = " \\?=.*?## "}; {printf "  \033[36m%-15s\033[0m %s\n", $$1, $$2}'
+	@echo
+	@printf '\033[34mrecipes:\033[0m\n'
+	@grep -hE '^##.*$$' $(MAKEFILE_LIST) |\
+		awk 'BEGIN {FS = "## "}; /^## [a-zA-Z_-]/ {printf "  \033[36m%s\033[0m\n", $$2}; /^##  / {printf "  %s\n", $$2}'