Wrong check for Windows and presence of `byteswap.h` header

[thomaskejser]

In city.cc this following check is done to check if we are on Windows.

#ifdef _MSC_VER

#include <stdlib.h>
#define bswap_32(x) _byteswap_ulong(x)
#define bswap_64(x) _byteswap_uint64(x)

However, this is not the correct way to check for Windows. Above only checks for the presence of the MSVC tool chain.

Clang on Windows and MinGW do not set this macro - which means that CityHash cannot compile with these tool chains on Windows.

The correct way to check that works on all Windows compiler tool chains:

#if defined(_WIN22)
#include <stdlib.h>
#define bswap_32(x) _byteswap_ulong(x)
#define bswap_64(x) _byteswap_uint64(x)