fix a warning

include/macros.h

@@ -110,11 +110,13 @@
 
 // Envelopes are always stored as big endian, to match sequence files which are
 // byte blobs and can embed envelopes. Hence this byteswapping macro.
+#ifndef BSWAP16
 #if IS_BIG_ENDIAN
 #define BSWAP16(x) (x)
 #else
 #define BSWAP16(x) (((x) & 0xff) << 8 | (((x) >> 8) & 0xff))
 #endif
+#endif
 
 /**
  * (u8*) dl : Cast array down to u8's