[ruby/mmtk] Set MMTK_DEBUG in C compiler for debug builds

ruby/mmtk@12a3904b04

Author: peterzhu2118

gc/mmtk/extconf.rb

@@ -15,6 +15,10 @@
     MMTK_BUILD = debug
     LIBMMTK_RUBY = libmmtk_ruby.#$LIBEXT
     RUSTSRCS = #{rustsrcs.join(" \\\n\t   ")}
+
+    ifeq ($(MMTK_BUILD), debug)
+    CPPFLAGS += -DMMTK_DEBUG
+    endif
     MAKEFILE
   ]
 end