So the basic way of how I imagined this should work is:

a) run qmake to find out the place of the Qt installation, in particular
the include, lib and plugin dirs and most importantly the mkspecs dir
and which mkspec to use (possibly use-configurable).
b) read and parse the .prf files from mkspecs/features, usually starting with
qt_config.prf this will declare a lot of the QMake functions and they're
implemented with qmake itself
c) read and parse the files for the mkspec (qmake.conf, all the included
files), this should produce a couple base variables
d) start with the top-level qmake file from the project (this might
again need user-specification via gui which file should be used)
e) during parsing, evaluate all the known functions, so we automatically
dive into include() directives, can do variable substitution and also
the list-stuff qmake supports

Thats the theory, whats there right now is a full qmake parser (no known
bugs or missing features from qmake) and the start of some basic
evaluation. I've actually started with the variable substitution.