Improve Makefile to hack crystal

- Use spaces instead of tab as an indent of no command line
- Add "optional variables" and "recipes" section for help outputs
- Output colored llvm-config information (this is same color of
  ./bin/crystal's "Using compiled compiler at" information)

Makefile

@@ -1,5 +1,24 @@
 -include Makefile.local # for optional local options e.g. threads
 
+# Recipes for this Makefile
+
+## Build the compiler
+##   $ make
+## Clean up built files then build the compiler
+##   $ make clean crystal
+## Build the compiler in release mode
+##   $ make crystal release=1
+## Run all specs in verbose mode
+##   $ make specs verbose=1
+
+LLVM_CONFIG ?= ## llvm-config command path to use
+
+release ?= ## Compile in release mode
+stats ?=   ## Enable statistics output
+threads ?= ## Maximum number of threads to use
+debug ?=   ## Add symbolic debug info
+verbose ?= ## Run specs in verbose mode
+
 O := .build
 SOURCES := $(shell find src -name '*.cr')
 SPEC_SOURCES := $(shell find spec -name '*.cr')
@@ -8,12 +27,14 @@ VERBOSE := $(if $(verbose),-v )
 EXPORTS := $(if $(release),,CRYSTAL_CONFIG_PATH=`pwd`/src)
 SHELL = bash
 LLVM_CONFIG_FINDER := \
-	[ -n "$(LLVM_CONFIG)" ] && command -v "$(LLVM_CONFIG)" || \
-	command -v llvm-config-3.9 || command -v llvm-config39 || (command -v llvm-config > /dev/null && (case "$(llvm-config --version)" in 3.9*) command -v llvm-config;; *) false;; esac)) || \
-	command -v llvm-config-3.8 || command -v llvm-config38 || (command -v llvm-config > /dev/null && (case "$(llvm-config --version)" in 3.8*) command -v llvm-config;; *) false;; esac)) || \
-	command -v llvm-config-3.6 || command -v llvm-config36 || \
-	command -v llvm-config-3.5 || command -v llvm-config35 || \
-	command -v llvm-config
+  [ -n "$(LLVM_CONFIG)" ] && command -v "$(LLVM_CONFIG)" || \
+  command -v llvm-config-3.9 || command -v llvm-config39 || \
+    (command -v llvm-config > /dev/null && (case "$(llvm-config --version)" in 3.9*) command -v llvm-config;; *) false;; esac)) || \
+  command -v llvm-config-3.8 || command -v llvm-config38 || \
+    (command -v llvm-config > /dev/null && (case "$(llvm-config --version)" in 3.8*) command -v llvm-config;; *) false;; esac)) || \
+  command -v llvm-config-3.6 || command -v llvm-config36 || \
+  command -v llvm-config-3.5 || command -v llvm-config35 || \
+  command -v llvm-config
 LLVM_CONFIG := $(shell $(LLVM_CONFIG_FINDER))
 LLVM_EXT_DIR = src/llvm/ext
 LLVM_EXT_OBJ = $(LLVM_EXT_DIR)/llvm_ext.o
@@ -26,18 +47,28 @@ CXXFLAGS += $(if $(debug),-g -O0)
 ifeq (${LLVM_CONFIG},)
   $(error Could not locate llvm-config, make sure it is installed and in your PATH, or set LLVM_CONFIG)
 else
-  $(info Using $(LLVM_CONFIG) [version=$(shell $(LLVM_CONFIG) --version)])
+  $(info $(shell printf '\033[33m')Using $(LLVM_CONFIG) [version=$(shell $(LLVM_CONFIG) --version)]$(shell printf '\033[0m'))
 endif
 
 .PHONY: all
-all: crystal
+all: crystal ## Build all files (currently crystal only) [default]
 
 .PHONY: help
 help: ## Show this help
+	@echo
 	@printf '\033[34mtargets:\033[0m\n'
 	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) |\
 		sort |\
 		awk 'BEGIN {FS = ":.*?## "}; {printf "  \033[36m%-15s\033[0m %s\n", $$1, $$2}'
+	@echo
+	@printf '\033[34moptional variables:\033[0m\n'
+	@grep -E '^[a-zA-Z_-]+ \?=.*?## .*$$' $(MAKEFILE_LIST) |\
+		sort |\
+		awk 'BEGIN {FS = " \\?=.*?## "}; {printf "  \033[36m%-15s\033[0m %s\n", $$1, $$2}'
+	@echo
+	@printf '\033[34mrecipes:\033[0m\n'
+	@grep -E '^##.*$$' $(MAKEFILE_LIST) |\
+		awk 'BEGIN {FS = "## "}; /^## [a-zA-Z_-]/ {printf "  \033[36m%s\033[0m\n", $$2}; /^##  / {printf "  %s\n", $$2}'
 
 .PHONY: spec
 spec: all_spec ## Run all specs