Small fix for compiler-rt on Solaris/Illumos

["C. Bergström"]

It was reviewed by nbjoerg on irc
---------
diff --git a/lib/int_endianness.h b/lib/int_endianness.h
index edb58c8..a64f926 100644
--- a/lib/int_endianness.h
+++ b/lib/int_endianness.h
@@ -19,13 +19,15 @@
  #if defined(__SVR4) && defined(__sun)
  #include <sys/byteorder.h>

-#if _BYTE_ORDER == _BIG_ENDIAN
+#if defined(_BIG_ENDIAN)
  #define _YUGA_LITTLE_ENDIAN 0
  #define _YUGA_BIG_ENDIAN    1
-#elif _BYTE_ORDER == _LITTLE_ENDIAN
+#elif defined(_LITTLE_ENDIAN)
  #define _YUGA_LITTLE_ENDIAN 1
  #define _YUGA_BIG_ENDIAN    0
-#endif /* _BYTE_ORDER */
+#else /* !_LITTLE_ENDIAN */
+#error "unknown endianness"
+#endif /* !_LITTLE_ENDIAN */

  #endif /* Solaris and AuroraUX. */