__builtin_bswap16 only found in gcc-4.5+.

https://bugs.freedesktop.org/show_bug.cgi?id=94570

Index: spice/macros.h
--- spice/macros.h.orig
+++ spice/macros.h
@@ -240,7 +240,7 @@
 
 /* Arch specific stuff for speed
  */
-#if defined (__GNUC__) && (__GNUC__ >= 4) && defined (__OPTIMIZE__)
+#if (defined(__clang__) || defined (__GNUC__) && __GNUC__ >= 4 && __GNUC_MINOR__ >= 5) && defined (__OPTIMIZE__)
 #  define SPICE_BYTESWAP16(val) __builtin_bswap16(val)
 #  define SPICE_BYTESWAP32(val) __builtin_bswap32(val)
 #  define SPICE_BYTESWAP64(val) __builtin_bswap64(val)
@@ -260,6 +260,13 @@
 #define SPICE_ENDIAN_LITTLE 4321
 #define SPICE_ENDIAN_BIG    1234
 #define SPICE_ENDIAN_PDP    2143
+
+#include <endian.h>
+#if BYTE_ORDER == LITTLE_ENDIAN
+#  define SPICE_ENDIAN SPICE_ENDIAN_LITTLE
+#elif BYTE_ORDER == BIG_ENDIAN
+#  define SPICE_ENDIAN SPICE_ENDIAN_BIG
+#endif
 
 /* gcc already defined these, use them */
 #if defined(__BYTE_ORDER__) && defined(__ORDER_LITTLE_ENDIAN__) \