Config: added two new options to configure local builds

This will allow to ease code modifications in packages and more
flexible build configurations for development tasks.

By default the packages will need to be manually downloaded in local/.
qt5webkit: enable local build support

If local build support is globally enabled then put your qt5webkit
folder in local/, do code modifications there and rebuild with make
qt5webkit-reconfigure.
Merge pull request #36 from Metrological/local-builds

Local builds support