C/C++ 跨平台时预处理判断平台环境

在编写跨平台的 C/C++ 时，需要判断平台的操作系统类型。通过预编译宏，区分各平台。

参考 How to detect reliably Mac OS X, iOS, Linux, Windows in C preprocessor? [duplicate]

注：

• WIN32在 win32 和 win64 下都会被定义，所以先判 WIN64

#if defined(WIN32) || defined(_WIN32) || defined(__WIN32__) || defined(__NT__)
   //define something for Windows (32-bit and 64-bit, this part is common)
   #ifdef _WIN64
      //define something for Windows (64-bit only)
   #else
      //define something for Windows (32-bit only)
   #endif
#elif __APPLE__
    #include <TargetConditionals.h>
    #if TARGET_IPHONE_SIMULATOR
         // iOS Simulator
    #elif TARGET_OS_IPHONE
        // iOS device
    #elif TARGET_OS_MAC
        // Other kinds of Mac OS
    #else
    #   error "Unknown Apple platform"
    #endif
#elif __linux__
    // linux
#elif __unix__ // all unices not caught above
    // Unix
#elif defined(_POSIX_VERSION)
    // POSIX
#else
#   error "Unknown compiler"
#endif