什么是C++中的编译器特性检测？

#include <iostream>
#include <optional>

#ifdef __has_include
#  if __has_include(<optional>)
#    if defined(__cpp_lib_optional) && __cpp_lib_optional >= 201606L
       // 使用 std::optional
       #include <optional>
       void use_optional() {
           std::optional<int> opt = std::nullopt;
           if (opt) {
               std::cout << "Optional has value: " << *opt << std::endl;
           } else {
               std::cout << "Optional is empty" << std::endl;
           }
       }
#    else
       // 不支持 std::optional，提供替代方案
       void use_optional() {
           int* opt = nullptr;
           if (opt) {
               std::cout << "Pointer has value: " << *opt << std::endl;
           } else {
               std::cout << "Pointer is null" << std::endl;
           }
       }
#    endif
#  else
       // <optional> 头文件不存在，提供替代方案
       void use_optional() {
           int* opt = nullptr;
           if (opt) {
               std::cout << "Pointer has value: " << *opt << std::endl;
           } else {
               std::cout << "Pointer is null" << std::endl;
           }
       }
#  endif
#else
       // __has_include 不支持，提供替代方案
       void use_optional() {
           int* opt = nullptr;
           if (opt) {
               std::cout << "Pointer has value: " << *opt << std::endl;
           } else {
               std::cout << "Pointer is null" << std::endl;
           }
       }
#endif

int main() {
    use_optional();
    return 0;
}