Change examples to use dynamic selection headers

Since they'd gotten out of sync a while ago, it seems.

Author: cpsauer

examples/dir.cpp

@@ -3,17 +3,7 @@
 #include <iostream>
 #include <string>
 
-#if defined(__cplusplus) && __cplusplus >= 201703L && defined(__has_include)
-#if __has_include(<filesystem>)
-#define GHC_USE_STD_FS
-#include <filesystem>
-namespace fs = std::filesystem;
-#endif
-#endif
-#ifndef GHC_USE_STD_FS
-#include <ghc/filesystem.hpp>
-namespace fs = ghc::filesystem;
-#endif
+#include <ghc/fs_std.hpp>
 
 template <typename TP>
 std::time_t to_time_t(TP tp)

examples/du.cpp

@@ -2,17 +2,7 @@
 #include <iomanip>
 #include <chrono>
 
-#if defined(__cplusplus) && __cplusplus >= 201703L && defined(__has_include)
-#if __has_include(<filesystem>)
-#define GHC_USE_STD_FS
-#include <filesystem>
-namespace fs = std::filesystem;
-#endif
-#endif
-#ifndef GHC_USE_STD_FS
-#include <ghc/filesystem.hpp>
-namespace fs = ghc::filesystem;
-#endif
+#include <ghc/fs_std.hpp>
 
 int main(int argc, char* argv[])
 {