* This seemed to cause crashes on some systems as libproxy links to libstdc++
and can then cause symbol conflicts between the static libstdc++ and the
system's libstdc++.
* libproxy itself cannot be statically linked, so the solution is either to
configure Qt without libproxy and break anyone who needs proxy support, or
stop statically linking libstdc++ and add a new dependency. At commit time
most distributions have at least g++-5's libstdc++ in their minspec so we can
require that - the main two that seem to fall short are Debian Jessie and
CentOS 7. Users on those systems will have to install a newer libstdc++ or
compile from source rather than using the binary releases.
baldurk