Our libraries have their own workspace folder that is in a folder called Libraries under the workspace folder

Code:
MainProjectFolder
    Libraries
        szLibXL
        szVPE
        ...
we do not share libraries between projects. Updating a library would cause all projects to be modified. This is ok if you had a few projects but not if you have a lot of them in different DF versions.
Each one would need to be tested and redeployed to clients etc.