Workaround for bad GENERIC_TARGET_LIBRARY

Originally https://github.com/AutonomyLab/ardrone_autonomy/pull/93

If `GENERIC_TARGET_LIBRARY` is empty at this point in the `make`, it is set to be `GENERIC_LIBRARY_TARGET_DIR/`. Later checks then see that `GENERIC_TARGET_LIBRARY` is populated, and attempt to consume it as if it were a library file (wich it is not, it is just a directory).

This patch keeps `GENERIC_TARGET_LIBRARY` empty if it was empty to begin with.

Example of build break: https://csc.mcs.sdsmt.edu/jenkins/job/ros-indigo-ardrone-autonomy_binaryrpm_heisenbug_x86_64/1/console