Support for pcre2

[dvzrv]

Hi! I maintain the package for this project on Arch Linux.

We're trying to drop pcre and move to pcre2. It would be great if rasqal could make use of pcre2 instead of pcre (we have a downstream ticket tracking this: https://bugs.archlinux.org/task/74998).

Alternatively, downstreams may use POSIX regex.h (as mentioned in above ticket). I don't really know if there are any strings attached doing so.

Either way, feedback from @dajobe would be highly appreciated 🙏

[dajobe]

I added an upstream support fix for PCRE2 at 4a2e3d2 however just for the record, POSIX regex.h is just fine as it's only using posix regex match & replace functionality.