libxl: fix build with glibc < 2.9

htobe*() and be*toh() don't exist there. While replacing the 32-bit
ones with hton() and ntoh() would be possible, there wouldn't be an
obvious replacement for the 64-bit ones. Hence just take what current
glibc (2.21) has (assuming __bswap_*() exists, which it does back to
at least 2.4 according to my checking).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Not sure whether I picked an appropriate header to place this in, or
an appropriate #ifdef to hook this onto. Hence the RFC.

--- a/tools/libxl/libxl_osdeps.h
+++ b/tools/libxl/libxl_osdeps.h
@@ -59,6 +59,42 @@ int asprintf(char **buffer, char *fmt, .
 int vasprintf(char **buffer, const char *fmt, va_list ap);
 #endif /*NEED_OWN_ASPRINTF*/
 
+#ifndef htobe32 /* glibc < 2.9 */
+# include <byteswap.h>
+
+# if __BYTE_ORDER == __LITTLE_ENDIAN
+#  define htobe16(x) __bswap_16(x)
+#  define htole16(x) (x)
+#  define be16toh(x) __bswap_16(x)
+#  define le16toh(x) (x)
+
+#  define htobe32(x) __bswap_32(x)
+#  define htole32(x) (x)
+#  define be32toh(x) __bswap_32(x)
+#  define le32toh(x) (x)
+
+#  define htobe64(x) __bswap_64(x)
+#  define htole64(x) (x)
+#  define be64toh(x) __bswap_64(x)
+#  define le64toh(x) (x)
+# else
+#  define htobe16(x) (x)
+#  define htole16(x) __bswap_16(x)
+#  define be16toh(x) (x)
+#  define le16toh(x) __bswap_16(x)
+
+#  define htobe32(x) (x)
+#  define htole32(x) __bswap_32(x)
+#  define be32toh(x) (x)
+#  define le32toh(x) __bswap_32(x)
+
+#  define htobe64(x) (x)
+#  define htole64(x) __bswap_64(x)
+#  define be64toh(x) (x)
+#  define le64toh(x) __bswap_64(x)
+# endif
+#endif
+
 #endif
 
 /*