allow library names as targets.

It was always pretty annoying that there was no easy target to build a library (eg. `make build/x86_64/bin/libblah.so`). This pull request rectifies that so you can now build a library by pre-pending `lib` to the library name declared in th make file (eg. `make libblah`). The lib prefix was added to avoid any name conflicts with executables (shouldn't really be an issue but better safe then sorry).