This follows the same logic as before with the added check that a libpnetcdf.*
file exists in the checked directory. The change in logic will now account for
situations where PNETCDF is set to a directory containing both lib/ and lib64/
directories and libpnetcdf.* exists in just the lib/ folder. Should libpnetcdf.*
exist in both directories, the lib64/ still maintains precedence.

As the netcdf library logic mirrors the pnetcdf one, it would suffer
from the same issues noted in 0ca4208 about lib/lib64 checking.
This updates the corresponding issue in the netcdf library detection
before it becomes an issue.