Merge pull request #651 from bovi/make-rake

Add Makefile for backward compatibility of shell build processes.

Makefile

+# mruby is using Rake (http://rake.rubyforge.org) as a build tool.
+# We provide a minimalistic version called minirake inside of our 
+# codebase.
+
+RAKE = ./minirake
+
+.PHONY : all
+all :
+	$(RAKE)
+
+.PHONY : test
+test :
+	$(RAKE) test
+
+.PHONY : clean
+clean :
+	$(RAKE) clean
+
+.PHONY : showconfig
+showconfig :
+	$(RAKE) showconfig