Unfortunately we will have to revert this it seems,

CMakeFiles/kcm_lookandfeel.dir/kcm.cpp.o: In function `KCMLookandFeelFactory::KCMLookandFeelFactory()': 
kcm.cpp:(.text+0x2b): undefined reference to `vtable for KCMLookandFeelFactory'                         
CMakeFiles/kcm_lookandfeel.dir/kcm.cpp.o: In function `KCMLookandFeelFactory::~KCMLookandFeelFactory()':
kcm.cpp:(.text+0xad): undefined reference to `vtable for KCMLookandFeelFactory'                         
collect2: error: ld returned 1 exit status

It results in error above

Could not reproduce that error locally, and build.kde.org had also not reported any problems. But given it failed for at least two people and the neon CI (https://build.neon.kde.org/job/xenial_unstable_plasma_plasma-desktop_bin_amd64/528/consoleFull) there might be some other problem hidden somewhere.

For what I looked, all generated Make rules also looked fine here (clean build), with expected conditions. grep DEFINE_NO_PLUGIN . -R only got me hits for the lookandfeeltool targets, especially

./CMakeFiles/lookandfeeltool_autogen.dir/AutogenInfo.cmake:set(AM_MOC_DEFINITIONS "DEFINE_NO_PLUGIN;KCOREADDONS_LIB;KGUIADDONS_LIB;QT_CORE_LIB;QT_DBUS_LIB;QT_DISABLE_DEPRECATED_BEFORE=0;QT_GUI_LIB;QT_NETWORK_LIB;QT_NO_URL_CAST_FROM_STRING;QT_PRINTSUPPORT_LIB;QT_QML_LIB;QT_QUICKWIDGETS_LIB;QT_QUICK_LIB;QT_WIDGETS_LIB;QT_X11EXTRAS_LIB;QT_XML_LIB;TRANSLATION_DOMAIN=\"kcm_lookandfeel\";_GNU_SOURCE;_LARGEFILE64_SOURCE")

By the error reported it seems though the generated kcms/lookandfeel/kcm_lookandfeel_autogen/include/kcm.moc was without the code of the plugin factoy for whatever reason, where it should have it though. No idea why by just looking at the code.

Given I have no access to neon builds and also reached frustration level here :) I am now simply proposing D10607 as an alternative solution without a preprocessor definition, that should avoid any possible confusion cmake/automoc might have here.