depends: Use pic for all linux dependencies

This avoids textrels, and matches previous gitian behavior.
depends: Add a package for qt4.6. Linux uses it by default.

We're not ready to switch to a static qt5 for Linux yet due to missing plugin
support. This adds a recipe for building a shared qt4 that we build and link
against, but don't distribute.

make USE_LINUX_STATIC_QT5=1 can be used to build static qt5 as before.
gitian: descriptors overhaul

Descriptors now make use of the dependencies builder, so results are cached.
A very new version (>= e9741525c) of Gitian should be used in order to take
advantage of caching.
gitian: quick docs update
travis: let travis use shared libs for tests