Qt 5.5.0 moc: explicitly need to define QT_VERSION

Hi,

I am creating a Qt plugin on Mac and using Qt 5.5.0.
This conforms to the standard way(s) of creating a Qt plugin.
When running moc on the plugin source, I do not get the expected
QT_MOC_EXPORT_PLUGIN(..., ...) /* name omitted */
line that should be generated by moc to create a plugin.
However if on the moc command line I pass, -DQT_VERSION=0x050000, this line does get generated.

However on Windows it works as expected, and -DQT_VERSION=0x050000 is not needed.

Why is this difference in moc behaviour between Windows and Mac?
Shouldn't moc 'know' the Qt Version without being explicitly told?