With the Boost.Regex to ehader-only library, the declaration
of a mutex that should have been changed from boost::static_mutex
to std::mutex was left behind. This was preventing regex from
being built for older arm platforms [1]
[1]: https://github.com/openwrt/packages/issues/15725
Signed-off-by: Carlos Miguel Ferreira <carlosmf.pt@gmail.com>