Build fails on macOS 10.12 (Sierra) with LLVM Clang 8.0.0

[cstrotm]

In file included from session.cc:28:
In file included from ../../../ext/asio/asio.hpp:49:
In file included from ../../../ext/asio/asio/ip/basic_resolver.hpp:23:
In file included from ../../../ext/asio/asio/ip/basic_resolver_query.hpp:21:
../../../ext/asio/asio/ip/resolver_query_base.hpp:114:3: error: all paths through this function will call itself
[-Werror,-Winfinite-recursion]
{
^
1 error generated.

[cstrotm]

file is copied from an older BOOST version.

Current version of the file from BOOST is at
http://www.boost.org/doc/libs/1_63_0/boost/asio/ip/resolver_query_base.hpp

fix:

friend flags operator~(flags x)
{
return static_cast(~static_cast(x));
}