We use something with the _ext or _EXT suffix (i.e MyProject_EXT) to signify that it is external to our project to store the source code of external packages that we link against.
I agree with Peter. The external libraries should be not be built into the system root as they may cause conflicts. I'd build them in their directory and then install them into a /lib directory (or maybe /extlib) that is unique to your application and link to them there.
