use C style comments instead of C++ style comments

According to CONTRIBUTING.md,

Don't use C++ style comments

 /* This is the prefered comment style */

Use C++ style comments only for temporary comment e.g. commenting out some code lines.

include/mruby/variable.h

@@ -16,9 +16,9 @@ typedef struct global_variable {
     mrb_value *data;
     mrb_value (*getter)(void);
     void  (*setter)(void);
-    //void  (*marker)();
-    //int block_trace;
-    //struct trace_var *trace;
+    /* void  (*marker)(); */
+    /* int block_trace; */
+    /* struct trace_var *trace; */
 } global_variable;
 
 struct global_entry {