Fixed broken compile on 1.9.3 due to missing rb_check_arity (added in 2.0.0).

Author: jdantonio

ext/concurrent_ruby_ext/atomic_boolean.c

@@ -2,6 +2,7 @@
 
 #include "atomic_boolean.h"
 #include "atomic_reference.h"
+#include "common.h"
 
 void atomic_boolean_mark(void *value) {
   rb_gc_mark_maybe((VALUE) value);

ext/concurrent_ruby_ext/atomic_fixnum.c

@@ -2,6 +2,7 @@
 
 #include "atomic_fixnum.h"
 #include "atomic_reference.h"
+#include "common.h"
 
 void atomic_fixnum_mark(void *value) {
   rb_gc_mark_maybe((VALUE) value);

ext/concurrent_ruby_ext/common.h

@@ -0,0 +1,11 @@
+#ifndef rb_check_arity
+
+// https://github.com/ruby/ruby/blob/ruby_2_0_0/include/ruby/intern.h
+// rb_check_arity was added in Ruby 2.0
+
+#define rb_check_arity(argc, min, max) do { \
+  if (((argc) < (min)) || ((argc) > (max) && (max) != UNLIMITED_ARGUMENTS)) \
+  rb_error_arity(argc, min, max); \
+} while(0)
+
+#endif