switch back to boost::filesystem to avoid the requirement of c++17

webcc/response_builder.h

@@ -4,6 +4,8 @@
 #include <string>
 #include <vector>
 
+#include "boost/filesystem/path.hpp"
+
 #include "webcc/request.h"
 #include "webcc/response.h"
 
@@ -94,7 +96,7 @@ public:
 
   // Use the file content as body.
   // NOTE: Error::kFileError might be thrown.
-  ResponseBuilder& File(const std::filesystem::path& path,
+  ResponseBuilder& File(const boost::filesystem::path& path,
                         bool infer_media_type = true,
                         std::size_t chunk_size = 1024);