gitian: build against Qt 4.6

Should make it possible to run the resulting GUI executable on
Linux distributions that use Qt 4.6, such as Debian Wheezy and Tails.

Builds a mini-SDK for building against Qt 4.6. This includes the headers
as well as host utilities such as lrelease, qrc and moc.

This speeds up the gitian build a bit - libqt4-dev pulled in a lot of packages,
and is no longer needed as this provides a replacement of our own.

Note: This does not replace the Qt build with at static library. After this
commit we still build dynamically against the system Qt library. The only
difference is that compatibility with an older version is maintained. This
loses minor GUI functionality (such as setPlaceholderText) but still
allows integration into the window management of the host OS, unlike
when statically linking.