Fallback to Boost.Regex if <regex> is broken

A large number of g++ versions ship a <regex> that declares all of the
required functions, but don't actually implement them, making using them
result in a linker error.

Fallback to Boost.Regex if the host C++11 <regex> implementation is
broken; the interface is the same anyway, only differing in the
containing namespace.

Unfortunately, Boost.Regex is not a header-only library, but this is not
a big deal because all major Linux distributions ship it, and Visual
Studio implements <regex> since 2010 (the oldest version we still
support).