proper prefix handling

This pull request does:

1) add global.h, containing PREFIX definition, remove all in-submodule definitions, use this global one instead (makes it straight-forward to change the prefix)

2) check for PREFIX and LIBPREFIX in qmake .pro files, allowing to change installation prefixes. If those environment variables are not passed to qmake, they will be set to /usr/local and /usr/local/lib. Example: qmake PREFIX=/usr LIBPREFIX=/usr/lib/x86_64-linux-gnu

3) update debian packaging scripts accordingly