Wrong byte-order detection on Mac OS X

`libfluid_msg` use this method to check host system endianness:

``` c++
#if __BYTE_ORDER == __BIG_ENDIAN
...
#else
...
#endif
```

However, both this non-standard preprocessor variables isn't defined on Mac OS X (tested on 10.9.5) and `#if` statement becomes true (big endian) on x86 system.
