Avoid using the moc from Qt 5 with Qt 4 based targets. Moc generates
a version check to ensure that such generated code does not compile.

The Qt4And5Automoc unit test should have been testing this, but it
was not because the test was broken. In that unit test, moc was run
on trivial files which have no significant content, and in particular
no Q_OBJECT macro.  Therefore moc was generating empty files which
do not even contain the version check.  Fix this by generating files
for input to moc at cmake time.