QMakeProject: Fix inconsistent isAbsolutePath

The QMakeEvaluator uses it's own more accurate version of
isAbsolutePath. If we pass in a path that qt thinks is
absolute, e.g. "C:dev", but the qmake evaluator disagrees,
creator crashes. Import these functions from the qmake
evaluator into Utils::FileUtils and use them to prepare
the builddirectory.

Task-number: QTCREATORBUG-12034
Change-Id: Ida28688558f3186db25e3ab73bd39cabc91c1ff0
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: David Schulz <david.schulz@digia.com>