[vcxproj] mocables are always rebuilt even if nothing changed

Using snapshot http://download.qt.io/snapshots/qt/5.8/5.8.0-rc/689/

moc_predefs.h is unconditionally regenerated every time a vcxproj project is built. Moc custom build steps have a dependency on moc_predefs.h. This means that all moc-generated files are regenerated and rebuilt every time a project is built, even if no sources or headers have been changed since the last build. This is a significant impediment to the edit-build-debug cycle.