Avoid potential redifinitions of HAVE_REMAP

Author: jonringer

ext/mruby_engine/dlmalloc_config.h

@@ -2,7 +2,11 @@
 #define MRUBY_ENGINE_DLMALLOC_CONFIG_H
 
 #define ONLY_MSPACES 1
+
+// Ruby may also define this
+#ifndef HAVE_MREMAP
 #define HAVE_MREMAP 0
+#endif
 //#define FOOTERS 1
 
 #include "host.h"