In this blog, I will explain how you can refer external dll in your SharePoint solution to make it work whenever this dll is not there in the Global Assembly Cache. So, the solution itself must contain the dll. To add dll into your SharePoint solution, given below are the steps.
Once you are done with the above steps, verify that WSP is containing that dll.
In my case, I have renamed it from EmbedExternalDLLs.wsp file to EmbedExternalDLLs.cab and extracted it to the folder. In that folder, you will find that added dll.
I have attached my demo solution EmbedExternalDLLs for reference.