Compiling a dll project is usually a little more complicated than can be provided in a forum response.
Borland normally provides very good documentation, tutorials and samples with their products. If you've never built a DLL before, I'd suggest you try out the samples and tutorials provided with bcppbuilder before you tackle Half-Life.
However, I found the following in less than a minute on the Borland site.
The Object Repository contains forms, dialog boxes, data modules, wizards, DLLs, sample applications, and other items that can simplify development. Choose File|New|Other to display the New Items dialog box when you begin a project.The New Items dialog box is the same as the Object Repository. Check the Repository to see if it contains an object that resembles one you want to create.
You knew this was coming, right? RTFM.
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