Depends on deprecated pcre library

It looks like this depends on pcre "v1", when pcre2 is out: https://www.pcre.org/

pcre v1 is deprecated, so it seems that switching over would be wise. v2 has been out since 2015.

I'm not sure if this is actively developed or not. There's talks of a fork (#1515).

Not sure how hard this would be to do. ag is a great piece of software, though!