book: add C++17 __has_include preprocessor operator

Add a 2.7 subsection on __has_include for portable conditional header
inclusion, with a compiled example (code/2/2.27). Refs #1

Author: changkun

book/en-us/02-usability.md

@@ -1179,6 +1179,21 @@ static_assert(!is_signed_integral<unsigned>);
 static_assert(std::negation_v<std::is_floating_point<int>>);
 ```
 
+### `__has_include`
+
+C++17 standardized the preprocessor operator `__has_include`, which checks at compile time whether a header is available, enabling portable conditional inclusion:
+
+```cpp
+#if __has_include(<optional>)
+#  include <optional>
+#  define HAS_OPTIONAL 1
+#else
+#  define HAS_OPTIONAL 0
+#endif
+```
+
+This is very useful when supporting different standard-library versions, or providing a fallback when a feature might be missing.
+
 ## Conclusion
 
 This section introduces the enhancements to language usability in modern C++, which I believe are the most important features that almost everyone needs to know and use:

book/zh-cn/02-usability.md

@@ -1093,6 +1093,21 @@ static_assert(!is_signed_integral<unsigned>);
 static_assert(std::negation_v<std::is_floating_point<int>>);
 ```
 
+### `__has_include`
+
+C++17 标准化了预处理器运算符 `__has_include`，用于在编译期检查某个头文件是否可用，从而写出可移植的条件包含逻辑：
+
+```cpp
+#if __has_include(<optional>)
+#  include <optional>
+#  define HAS_OPTIONAL 1
+#else
+#  define HAS_OPTIONAL 0
+#endif
+```
+
+这在需要兼容不同标准库版本，或在某个特性可能缺失时提供回退实现的场景中非常有用。
+
 ## 总结
 
 本节介绍了现代 C++ 中对语言可用性的增强，其中笔者认为最为重要的几个特性是几乎所有人都需要了解并熟练使用的：

code/2/2.27.has.include.cpp

@@ -0,0 +1,27 @@
+//
+// 2.27.has.include.cpp
+// chapter 2 language usability
+// modern cpp tutorial
+//
+// created by changkun at changkun.de
+// https://github.com/changkun/modern-cpp-tutorial
+//
+
+#include <iostream>
+
+// C++17: __has_include is a preprocessor operator that checks whether a
+// header is available, enabling portable conditional inclusion.
+#if __has_include(<optional>)
+#  include <optional>
+#  define HAS_OPTIONAL 1
+#else
+#  define HAS_OPTIONAL 0
+#endif
+
+int main() {
+    std::cout << "<optional> available: " << HAS_OPTIONAL << std::endl;
+#if HAS_OPTIONAL
+    std::optional<int> o = 42;
+    std::cout << "value: " << o.value() << std::endl;
+#endif
+}