(maint) Ignore -Wunused-variable in Boost.Asio
(maint) Ignore -Wunused-variable in Boost.Asio

Boost.Asio copied functionality from Boost.System's error_code, but
doesn't copy it in providing a flag to skip it like
BOOST_SYSTEM_NO_DEPRECATED. So ignore unused-variable warnings generated
by Boost.Asio headers.

Due to bug #53431 in GCC 5.2, the warnings will still print, but won't
stop compilation. This may cause issues for AppVeyor when we update GCC.

MikaelSmith/cpp-pcp-client,MikaelSmith/cpp-pcp-client,MikaelSmith/cpp-pcp-client,puppetlabs/cpp-pcp-client,puppetlabs/cpp-pcp-client,MikaelSmith/cpp-pcp-client,MikaelSmith/cpp-pcp-client,puppetlabs/cpp-pcp-client,puppetlabs/cpp-pcp-client