Added flag to work as a library

src/main.cpp

@@ -2,6 +2,7 @@
 #include "CLI11.hpp"
 #include "Companion.h"
 
+#ifdef STANDALONE
 Companion* Companion::Instance;
 
 int main(int argc, char *argv[]) {
@@ -19,4 +20,5 @@ int main(int argc, char *argv[]) {
     Companion::Instance = new Companion(filename);
     Companion::Instance->Init();
     return 0;
-}
+}
+#endif