Add macro for filesystem.

Filesystem wasn't introduced until C++17, so including the library is
dodgy for different C++ versions, compiler and OSs. This should help,
but may be incomplete.

src/Scenario.cpp

@@ -11,11 +11,7 @@
 #include <sstream>
 #include <string>
 #include <sys/stat.h>
-#ifdef __APPLE__
-namespace fs = std::__fs::filesystem;
-#else
-namespace fs = std::experimental::filesystem;
-#endif
+#include "filesystem.hpp"
 
 bool IsPathExist(const std::string &s) {
   struct stat buffer;

src/filesystem.hpp

@@ -0,0 +1,11 @@
+// This is a workaround for the filesystem library to work for different OSs/compiler and C++ versions
+#ifdef __cpp_lib_filesystem // C++17
+    #include <filesystem>
+    namespace fs = std::filesystem;
+#elif __cpp_lib_experimental_filesystem // C++11
+    #include <experimental/filesystem>
+    namespace fs = std::experimental::filesystem;
+#else // MacOS C++11
+    #include <filesystem>
+    namespace fs = std::__fs::filesystem;
+#endif