Canonicalize inputs when filtering default system paths

Contrary to expectations, various <foo>-config tools sometimes spit out
denormalized paths, which breaks the text-based filtering, as it relies
on exact matches with normalized paths.

Change-Id: I0613ed24953a3bde19939d28d09572c88b43a361
Task-number: QTBUG-39216
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>