Add convenient makefile arguments for packaging (#414)

* Allow make all FLAGS="--no-debug --release" 

When packaging shards is usually compiled with --no-debug and --release. This change will allow using the makefile in those scripts.

Otherwise we need to manually use the crystal build or CRYSTAL_OPTS env var

* Add release debug static makefile arguments

Same as the ones found in crystal-lang/crystal

Makefile

@@ -1,7 +1,13 @@
 .POSIX:
 
+release ?=      ## Compile in release mode
+debug ?=        ## Add symbolic debug info
+static ?=       ## Enable static linking
+
 CRYSTAL ?= crystal
 SHARDS ?= shards
+override FLAGS += $(if $(release),--release )$(if $(debug),-d )$(if $(static),--static )
+
 SHARDS_SOURCES = $(shell find src -name '*.cr')
 MOLINILLO_SOURCES = $(shell find lib/molinillo -name '*.cr' 2> /dev/null)
 SOURCES = $(SHARDS_SOURCES) $(MOLINILLO_SOURCES)
@@ -23,7 +29,7 @@ clean: phony
 
 bin/shards: $(SOURCES) $(TEMPLATES) lib
 	@mkdir -p bin
-	$(CRYSTAL) build src/shards.cr -o bin/shards
+	$(CRYSTAL) build $(FLAGS) src/shards.cr -o bin/shards
 
 install: bin/shards phony
 	$(INSTALL) -m 0755 -d "$(BINDIR)" "$(MANDIR)/man1" "$(MANDIR)/man5"