PCRE dependency moved

[Georg Rudoy]

Hi there,

The PCRE regexp wrapper has been moved from Poshuku CleanWeb to the
util library so that other plugins could also use it.

This hardly changes anything for binary distros like Debian or
openSUSE, but Gentoo needs some care:
1. lc-poshuku's "pcre" USE flag can be removed, as well as the
dependency on the pcre library.
2. lc-core should get a new USE flag, "pcre", toggling the option
WITH_PCRE and the dependency on the pcre library.
3. I think it's a good idea if lc-poshuku[cleanweb] would ewarn
something like "Utility library is built without support for PCRE,
AdBlock+ support will be slow and inconclusive" if lc-core is built
without pcre.

--
Georg Rudoy