Fix library property for etterfilter and etterlog

If ettercap is being installed in a non-standard directory, **etterlog** and **etterfilter** doesn't find their library they're linked against:
```
boston:/home/koeppea/tmp/ettercap/bin# ./etterfilter
./etterfilter: error while loading shared libraries: libettercap.so.0: cannot open shared object file: No such file or directory
boston:/home/koeppea/tmp/ettercap/bin# 
```
This pull requests fixes this and makes the setting of the LD_LIBRARY_PATH environment variable obsolete in such cases.