Make sure the "==" operator is inline in arguments.hpp

src/headers/arguments.hpp

@@ -18,7 +18,7 @@ protected:
         return (not str.empty()) && longVersion == str && hasPrefix;
     }
 public:
-    bool operator ==(const string_view& it) { return is(it); }
+    inline bool operator ==(const string_view& it) { return is(it); }
     explicit Argument(string name): longVersion(move(name)) {}
 };