Need -pthread on some platforms if using an asio tcp_resolver

[mschout]

I have found out the hard way that the following code:

boost::asio::io_service io_service;
boost::asio::ip::tcp::resolver r(io_service);

will not link unless the -pthread flag is given on some platforms (at least FreeBSD and CentOS 7).

I am working around this with the following addition to boost.m4:

BOOST_ASIO_RESOLVER()

---------------------

tcp resolver requires the pthread flag for linking

BOOST_DEFUN([Asio_Resolver],
[AC_REQUIRE([_BOOST_PTHREAD_FLAG])
AC_SUBST([BOOST_ASIO_RESOLVER_LIBS], [$boost_cv_pthread_flag])
])

This allows me to set LDFLAGS (or libs) for programs that are using the resolver so they link correctly. Not sure if there is a better way to do this, but it solves the problem for me.