GCC: Only consider absolute paths for the system paths check
GCC: Only consider absolute paths for the system paths check

There is no reason to pay for the call to canonicalFilePath() if the
candidate is not an absolute path.

Change-Id: Ief39a72ef8967c1315fe6665e1b2795da168027e
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@qt.io>

qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs