Move version check to hpp from cpp

We should check MySQL version and define logical flags in hpp and use
them in cpp. It improves maintenability.

Some flags are used in two or more locations. If they are used in two
or more locations, we need to write the same version check conditions
there. It reduces maintenability:

Not good:

    #if MYSQL_VERSION_ID >= 50607
      // code1
    #endif
      // ...
    #if MYSQL_VERSION_ID >= 50607
      // code2
    #endif

Better:

    #if MYSQL_VERSION_ID >= 50607
    #  define LOGICAL_FLAG
    #endif

    #ifdef LOGICAL_FLAG
      // code1
    #endif
      // ...
    #ifdef LOGICAL_FLAG
      // code2
    #endif

ha_mroonga.cpp

@@ -3809,7 +3809,7 @@ int ha_mroonga::wrapper_open(const char *name, int mode, uint test_if_locked)
       DBUG_RETURN(HA_ERR_OUT_OF_MEM);
     }
     wrap_handler->init();
-#if MYSQL_VERSION_ID >= 50600
+#ifdef MRN_HANDLER_HAVE_SET_HA_SHARE_REF
     wrap_handler->set_ha_share_ref(&table->s->ha_share);
 #endif
     error = wrap_handler->ha_open(table, name, mode, test_if_locked);

ha_mroonga.hpp

@@ -91,6 +91,10 @@ extern "C" {
 #  define MRN_HANDLER_HAVE_GET_TABLESPACE_NAME
 #endif
 
+#if MYSQL_VERSION_ID >= 50607
+#  define MRN_HANDLER_HAVE_SET_HA_SHARE_REF
+#endif
+
 #if MYSQL_VERSION_ID >= 50500
 #  define MRN_TABLE_LIST_INIT_REQUIRE_ALIAS
 #endif