Create a "Be App" project. Add the libmdi.a static library to the project. Add the source directory of your Be MDI installation as access path. Finally add the BeOS shared library libstdc++.r4.so to the project.

(Note: Sometimes I had problems linking my projects, when libmdi.a wasn't the last entry in the project list. If you experience any problems try moving the libmid.a entry to the end of the list.)