link-grammar: explicitly disable Python bindings

In some OS environments (e.g., some Linux distros), Python in the base
userland was being detected, and bindings were generated, causing a
build failure due to PLIST mismatch. (Python bindings aren't required
for this package's intended dependencies in pkgsrc, and so aren't
meant to be included. If someone wants them, they should add them as a
non-default option.)


(gutteridge)