Fix compilation with GCC 4.7.4

[ccawley2011]

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

[lordmulder]

Maybe we should use a more generalized solution here?
https://github.com/lordmulder/faad2/commit/0af66230c2612d5c9848ad68e8789a4d1b4a77d1

static inline uint32_t bswap32(const uint32_t u32)
{
#ifndef WORDS_BIGENDIAN
#if defined (__GNUC__) && ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 3)))
    return __builtin_bswap32(u32);
#elif defined (_MSC_VER)
    return _byteswap_ulong(u32);
#else
    return (u32 << 24) | ((u32 << 8) & 0xFF0000) | ((u32 >> 8) & 0xFF00) | (u32 >> 24);
#endif
#else
    return u32;
#endif
}

static inline uint16_t bswap16(const uint16_t u16)
{
#ifndef WORDS_BIGENDIAN
#if defined (__GNUC__) && ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 8)))
    return __builtin_bswap16(u16);
#elif defined (_MSC_VER)
    return _byteswap_ushort(u16);
#else
    return (u16 << 8) | (u16 >> 8);
#endif
#else
    return u16;
#endif
}