This is a preparation step for 'make dist' target for subdir projects.
UnixMakefileGenerator needs these variables while extending
writeSubTargets() and writeDefaultVariables() for 'make dist'.

Partial cherry-pick of
https://qt.gitorious.org/qt/jpnurmi-qt/commit/8c4ef19



Task-number: QTBUG-21910
Change-Id: I02a616a98448bc3041ef0f4fd034bfb4c2199e41
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>