Automatically enable SHERPA_ENABLE_X when X_DIR provided
Currently we need to add two things for each option we want to enable in cmake, e.g.
cmake . -DSHERPA_ENABLE_OPENLOOPS=ON -DOPENLOOPS_DIR=/path/to/ol
This changes the behavior to automatically enable SHERPA_ENABLE_OPENLOOPS
when OPENLOOPS_DIR
was provided.