check for existence before execution of a tool
check for existence before execution of a tool

This is a work-around for QTBUG-5990.

Change-Id: I58f6f40cec7d9f5ac4bb90c1c2178b0e367fb1f6
Reviewed-by: Jake Petroules <2d627e1cecd5435752fa500bacbe05f7cc90c369@petroules.com>
Reviewed-by: Christian Kandeler <3aa99dcdd3f0cac61fb81c2a11771c0cc2497607@digia.com>

qt-labs/qbs,villytiger/qbs,villytiger/qbs,gatzka/qt-labs-qbs,villytiger/qbs,Distrotech/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,Distrotech/qbs,villytiger/qbs,Distrotech/qbs,Distrotech/qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,gatzka/qt-labs-qbs,Distrotech/qbs,Distrotech/qbs,qt-labs/qbs,Distrotech/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,villytiger/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,villytiger/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs