Implement libproxy support on linux

Configuration

• check in configure script for presence of required headers
• explicit enable/disable configure options
• don't automatically enable on windows or mac, prefer the system API

Basic implementation

• create a qnetworkproxy_libproxy file implementing the system proxy via this lib
• use this source file & link to required lib when configured

(note there is an old merge request in gitorious for this)

Advanced implementation

• libproxy has plugins for running PAC files
• ideally we want to use QtWebkit if app links to webkit, V8 if app links to declarative or QtScript if it links to that.
• we don't want libproxy to load yet another JS if we can avoid it
• plugin preferences are still prototype in libproxy, we might want to develop patches to libproxy as well.