Endian defines fix for macOS

It seems `__BYTE_ORDER`/`__LITTLE_ENDIAN`/`__BIG_ENDIAN` aren't defined
on macOS, so `if __BYTE_ORDER == __BIG_ENDIAN` was true which made macOS
take the big endian path *twice* (which cancelled out the big endian
conversion).

This makes util/endian.hpp define __LITTLE_ENDIAN__ or __BIG_ENDIAN__
everywhere, and errors if it can't be set.

llarp/net/ip.hpp

@@ -14,10 +14,10 @@
 // anything not win32
 struct ip_header
 {
-#if __BYTE_ORDER == __LITTLE_ENDIAN
+#ifdef __LITTLE_ENDIAN__
   unsigned int ihl : 4;
   unsigned int version : 4;
-#elif __BYTE_ORDER == __BIG_ENDIAN
+#elif defined(__BIG_ENDIAN__)
   unsigned int version : 4;
   unsigned int ihl : 4;
 #else

llarp/util/endian.hpp

@@ -94,6 +94,16 @@ be64toh(uint64_t big64);
 
 #endif
 
+#if !defined(__LITTLE_ENDIAN__) && defined(__BYTE_ORDER) \
+    && defined(__LITTLE_ENDIAN) && __BYTE_ORDER == __LITTLE_ENDIAN
+#define __LITTLE_ENDIAN__
+#elif !defined(__BIG_ENDIAN__) && defined(__BYTE_ORDER) \
+    && defined(__BIG_ENDIAN) && __BYTE_ORDER == __BIG_ENDIAN
+#define __BIG_ENDIAN__
+#elif !defined(__LITTLE_ENDIAN__) && !defined(__BIG_ENDIAN__)
+#error "Error: don't know which endian this is"
+#endif
+
 inline uint16_t
 buf16toh(const void *buf)
 {
@@ -193,7 +203,7 @@ htole64buf(void *buf, uint64_t big64)
 inline absl::uint128
 ntoh128(absl::uint128 i)
 {
-#if __BYTE_ORDER == __BIG_ENDIAN
+#ifdef __BIG_ENDIAN__
   return i;
 #else
   const auto loSwapped = htobe64(absl::Uint128Low64(i));