This function calculates the topological order of variables.
We will use it to determine which and in what order to link
module libraries.
The function is not tied to libraries/modules only, but requires
the variables to be ordered to have their dependencies in the
[prefix]<var>.depends subvariable.
Due to the recursive nature of the algorithm it was just much easier
to implement it directly in C++ rather than in a qmake-language
function.
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