Updates pdns-recursor to current stable 4.2.1. Also includes more complete fix
for boost.m4 BOOST::THREAD detection, removing dependency on boost-thread.

Signed-off-by: James Taylor <james@jtaylor.id.au>

Maintainer: me

This commit bumps the version of pdns-recursor to the latest 4.2.0. This release brings in mostly minor changes, with the full changelog available at https://doc.powerdns.com/recursor/changelog/4.2.html

Signed-off-by: James Taylor <james@jtaylor.id.au>

Maintainer: me

Compile tested: armv7l, OpenWRT SDK
Run tested: armv7l Linksys WRT1900ACS, OpenWrt SNAPSHOT, r9987-655fff1571 -
confirmed PowerDNS recursor links correctly against libraries and runs on my
target environment.

Description:
PowerDNS Recursor is a high-performance resolving name server, utilizing
multiple processor and including Lua scripting capabilities.

This commit includes the recursive nameserver

https://www.powerdns.com/recursor.html

Signed-off-by: James Taylor <james@jtaylor.id.au>