get header-only library dependencies using git submodules

gateway/httpgateway.h

@@ -1,6 +1,6 @@
 #ifndef HTTPGATEWAY_H
 #define HTTPGATEWAY_H
-#include "httplib.h"
+#include <httplib.h>
 #include "gatewayinterface.h"
 #include "../requestworker.h"
 #include "../request.h"