Build failure with asio-1.14.0

Abiword does not build against standalone asio-1.14.0, as reported by gentoo: https://bugs.gentoo.org/700852

In file included from ServiceUnixAccountHandler.h:23,
                 from ServiceUnixAccountHandler.cpp:20:
./../../../backends/service/xp/ServiceAccountHandler.h: In member function ‘void ServiceAccountHandler::_send(boost::shared_ptr<X>, RealmBuddyPtr)’:
./../../../backends/service/xp/ServiceAccountHandler.h:152:28: error: ‘error’ is not a member of ‘asio::placeholders’
        asio::placeholders::error, asio::placeholders::bytes_transferred, recipient,

I was able to reproduce the issue and the fix is to add -DASIO_ENABLE_BOOST to CXXFLAGS. The reason is most likely this commit:

https://github.com/chriskohlhoff/asio/commit/265e75cdbba739b1b1e3a46b283eaee3b9c60fe8

I am not sure where the best place to put this extra flag would be hence no merge request.