Add bad GENERIC_TARGET_LIBRARY workaround

This bug is preventing builds on Fedora.

What is happening, is that `GENERIC_TARGET_LIBRARY` is empty and `lib.makefile` prefixes that with `$(GENERIC_LIBRARY_TARGET_DIR)`, which `generic.makefile` then tries to build as a library target.

Obviously this fails, because `GENERIC_TARGET_LIBRARY` is a path to a folder.

I couldn't tell you why this isn't happening on other distros, but I tested the fix on Ubuntu 12.04 and it seemed to not cause a regression.