cross compile Qt build environment targeting linux, mac, windows on linux

We require a build platform for Qt targeting linux, mac and windows on debian/ubuntu linux. We don't use the Qt IDE tools and prefer to build from the command line. We'd prefer the outcome of this project to be a script that can be run on linux to download, build and install the cross compilers, Qt, and other prerequisites into a nominated prefix within the user's home directory. If this is too problematic a second choice would be a document describing in detail each of the steps needed to do this by hand. Candidates are requested to advise in their bid what option they will take. The project also produces a trivial Qt hello world type program demonstrating how static binaries can be built for each of the three targets.