automoc does not generate moc_predefs.h
When building a Qt application on i686, moc causes:
moc_iconapplet_FERWDM656PUOUJ.cpp:128:21: error: 'class IconApplet' has no member named 'open64'; did you mean 'open'?
This is due to moc not defining
_GNUC_ >= 2, so it emits code that does not match the header.
This issue is easily reproducable in a i686 build of kget or plasma-workspace.
This bug is fixed in Qt 5.8 as it dumps the list of compiler internal macros into a moc_predefs.h file but CMake does not do that: https://bugreports.qt.io/browse/QTBUG-57796