Add default makefile for Gems

mrbgems/Makefile4gem

+# This is the default Makefile integrated
+# by each Gem. It integrates important constants
+# for usage inside of a Gem.
+
+# mruby src root
+MRUBY_ROOT := ../../../
+
+# Tools
+CC := gcc
+RM := rm -f
+AR := ar
+
+SRC_DIR := src
+
+INCLUDES := -I$(SRC_DIR) -I$(MRUBY_ROOT)include -I$(MRUBY_ROOT)src -I.
+CFLAGS := $(INCLUDES) -O3 -g -Wall -Werror-implicit-function-declaration
+
+# LIBR can be manipulated with command line arguments
+ifeq ($(strip $(LIBR)),)
+  # default mruby library
+  LIBR := $(MRUBY_ROOT)lib/libmruby.a
+endif
+
+# Default rules which are calling the
+# gem specific gem-all and gem-clean
+# implementations of a gem
+
+.PHONY : all
+all : gem-info gem-all
+	@echo "Gem '$(GEM)' is done"
+
+gem-info:
+	@echo "Building Gem '$(GEM)'"
+
+.PHONY : clean
+clean : gem-clean
+	@echo "Gem '$(GEM)' is clean"

mrbgems/g/hello_world/Makefile

-GEMNAME := hello_world
-MRUBY_ROOT = ../../../
+include ../../Makefile4gem
 
-INCLUDES = -I$(MRUBY_ROOT)include -I$(MRUBY_ROOT)src -I.
-CFLAGS = $(INCLUDES) -O3 -g -Wall -Werror-implicit-function-declaration
-RM_F := rm -f
-AR := ar
+GEM := hello_world
 
-ifeq ($(strip $(LIBR)),)
-  # default mruby library
-  LIBR = $(MRUBY_ROOT)lib/libmruby.a
-endif
+GEM_C_FILES := $(wildcard $(SRC_DIR)/*.c)
+GEM_OBJECTS := $(patsubst %.c, %.o, $(GEM_C_FILES))
 
-.PHONY : all
-all : src/hello_world.o
-	@$(AR) rs $(LIBR) src/hello_world.o
-	@echo "Gem '$(GEMNAME)' is done"
+gem-all : $(GEM_OBJECTS)
+	$(AR) rs $(LIBR) $<
 
-src/hello_world.o : src/hello_world.c
-	@gcc -c $(CFLAGS) src/hello_world.c -o src/hello_world.o
-
-.PHONY : clean
-clean :
-	-$(RM_F) src/*.o
-	@echo "Gem '$(GEMNAME)' is clean"
+gem-clean :
+	-$(RM) $(GEM_OBJECTS)

mrbgems/g/hello_world/README.md

-mruby-md5
+hello world
 =========
 
-MD5 digest function.
-
-This library comes original from mattn (http://github.com/mattn/mruby-md5)
+This is an example gem which implements just one method +HW.say+ in a C extension.