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 theming and window management of the host OS, unlike when statically linking.

Test build:
https://download.visucore.com/bitcoin/linux-gitian-1b2454a.tar.gz (sigs: https://download.visucore.com/bitcoin/linux-gitian-1b2454a.tar.gz.sig )