Fixed __builtin_bswap16 not being available on GCC <4.8

See e.g. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52624

Change-Id: Idae99196c47e015a2ba2416aaf1526f77498ce5d

include/flatbuffers/flatbuffers.h

@@ -153,32 +153,34 @@ inline void EndianCheck() {
 
 template<typename T> T EndianSwap(T t) {
   #if defined(_MSC_VER)
-    #pragma push_macro("__builtin_bswap16")
-    #pragma push_macro("__builtin_bswap32")
-    #pragma push_macro("__builtin_bswap64")
-    #define __builtin_bswap16 _byteswap_ushort
-    #define __builtin_bswap32 _byteswap_ulong
-    #define __builtin_bswap64 _byteswap_uint64
+    #define FLATBUFFERS_BYTESWAP16 _byteswap_ushort
+    #define FLATBUFFERS_BYTESWAP32 _byteswap_ulong
+    #define FLATBUFFERS_BYTESWAP64 _byteswap_uint64
+  #else
+    #if defined(__GNUC__) && __GNUC__ * 100 + __GNUC_MINOR__ < 408
+      // __builtin_bswap16 was missing prior to GCC 4.8.
+      #define FLATBUFFERS_BYTESWAP16(x) \
+        static_cast<uint16_t>(__builtin_bswap32(static_cast<uint32_t>(x) << 16))
+    #else
+      #define FLATBUFFERS_BYTESWAP16 __builtin_bswap16
+    #endif
+    #define FLATBUFFERS_BYTESWAP32 __builtin_bswap32
+    #define FLATBUFFERS_BYTESWAP64 __builtin_bswap64
   #endif
   if (sizeof(T) == 1) {   // Compile-time if-then's.
     return t;
   } else if (sizeof(T) == 2) {
-    auto r = __builtin_bswap16(*reinterpret_cast<uint16_t *>(&t));
+    auto r = FLATBUFFERS_BYTESWAP16(*reinterpret_cast<uint16_t *>(&t));
     return *reinterpret_cast<T *>(&r);
   } else if (sizeof(T) == 4) {
-    auto r = __builtin_bswap32(*reinterpret_cast<uint32_t *>(&t));
+    auto r = FLATBUFFERS_BYTESWAP32(*reinterpret_cast<uint32_t *>(&t));
     return *reinterpret_cast<T *>(&r);
   } else if (sizeof(T) == 8) {
-    auto r = __builtin_bswap64(*reinterpret_cast<uint64_t *>(&t));
+    auto r = FLATBUFFERS_BYTESWAP64(*reinterpret_cast<uint64_t *>(&t));
     return *reinterpret_cast<T *>(&r);
   } else {
     assert(0);
   }
-  #if defined(_MSC_VER)
-    #pragma pop_macro("__builtin_bswap16")
-    #pragma pop_macro("__builtin_bswap32")
-    #pragma pop_macro("__builtin_bswap64")
-  #endif
 }
 
 template<typename T> T EndianScalar(T t) {