Remove usage of `which` in configure

[vnmabus]

When trying to install the aspell-en dictionary inside a Docker container, I had an error in the configure script because it tries to use the which command, which is not present in Docker by default. My proposal is to change this usage and use the command -v incantation instead, in a manner similar to what Apache did here.

Please @kevina forgive me if this is not the correct place for such a suggestion. I tried to pinpoint the origin of that configure script and the official webpage seemed to indicate that this is the tool used for packaging the dictionaries, and thus the origin of that configure script. However it seems like this tool has not be touched for years, so maybe the current packaging is done differently. In that case I would be glad if you could point me to the right place.

[kevina]

This is the correct place to fix the script. However the dictionaries will also need to be updated to use the newer script so an issue should also be filed at https://github.com/en-wl/wordlist/issues.

The dictionary has not been updated in a while, but there should be some sort of release this year.

[vnmabus]

I added a PR to fix it in #4.