Precompiled headers was failing in QtCreator

bugs, errors
#1

I tried setting precompileOfMain to true, but it was failing. I had to add /libs in ofApp.qbs:181 like this:

FileInfo.joinPaths(parent.of_root, '/libs/openFrameworks/ofMain.h'),

Then precompiled headers is shown on the left panel in QtCreator. Is that something specific to my system or should I send a PR?