Fix already-defined compiler warnings by adding some #ifndef in m4/byteorder.m4

m4/byteorder.m4

@@ -219,10 +219,18 @@ if test "$HAVE_LE32TOH" != "1"; then
 /* Define aliases for the standard byte swapping macros */
 /* Arguments to these macros must be properly aligned on natural word */
 /* boundaries in order to work properly on all architectures */
+#ifndef htobe16
 #define htobe16(x) htons(x)
+#endif
+#ifndef htobe32
 #define htobe32(x) htonl(x)
+#endif
+#ifndef be16toh
 #define be16toh(x) ntohs(x)
+#endif
+#ifndef be32toh
 #define be32toh(x) ntohl(x)
+#endif
 
 #define HTOBE16(x) (x) = htobe16(x)
 #define HTOBE32(x) (x) = htobe32(x)
@@ -234,13 +242,25 @@ EOF
  if test $ac_cv_c_bigendian = yes; then
   cat >> "$ac_byteorder" << EOF
 /* Define our own extended byte swapping macros for big-endian machines */
+#ifndef htole16
 #define htole16(x)      swap16(x)
+#endif
+#ifndef htole32
 #define htole32(x)      swap32(x)
+#endif
+#ifndef le16toh
 #define le16toh(x)      swap16(x)
+#endif
+#ifndef le32toh
 #define le32toh(x)      swap32(x)
+#endif
 
+#ifndef htobe64
 #define htobe64(x)      (x)
+#endif
+#ifndef be64toh
 #define be64toh(x)      (x)
+#endif
 
 #define HTOLE16(x)      (x) = htole16(x)
 #define HTOLE32(x)      (x) = htole32(x)
@@ -254,12 +274,24 @@ EOF
  else
   cat >> "$ac_byteorder" << EOF
 /* On little endian machines, these macros are null */
+#ifndef htole16
 #define htole16(x)      (x)
+#endif
+#ifndef htole32
 #define htole32(x)      (x)
+#endif
+#ifndef htole64
 #define htole64(x)      (x)
+#endif
+#ifndef le16toh
 #define le16toh(x)      (x)
+#endif
+#ifndef le32toh
 #define le32toh(x)      (x)
+#endif
+#ifndef le64toh
 #define le64toh(x)      (x)
+#endif
 
 #define HTOLE16(x)      (void) (x)
 #define HTOLE32(x)      (void) (x)
@@ -269,8 +301,12 @@ EOF
 #define LE64TOH(x)      (void) (x)
 
 /* These don't have standard aliases */
+#ifndef htobe64
 #define htobe64(x)      swap64(x)
+#endif
+#ifndef be64toh
 #define be64toh(x)      swap64(x)
+#endif
 
 #define HTOBE64(x)      (x) = htobe64(x)
 #define BE64TOH(x)      (x) = be64toh(x)