Lower priority on build

This should prevent slowdown of the IDE (and the rest of the system) while
building with all the CPU cores.

Fixes: QTCREATORBUG-5155
Change-Id: Icaadc53958f2d8e918035463e3c9344c91235615
Reviewed-by: hjk <hjk@qt.io>

src/plugins/qmakeprojectmanager/qmakestep.cpp

@@ -76,6 +76,7 @@ QMakeStep::QMakeStep(BuildStepList *bsl) : AbstractProcessStep(bsl, Constants::Q
 {
     //: QMakeStep default display name
     setDefaultDisplayName(tr("qmake"));
+    setLowPriority();
 }
 
 QmakeBuildConfiguration *QMakeStep::qmakeBuildConfiguration() const