There are these 3 lines in qttools/src/assistant/CMakeLists.txt:

 
qt_internal_set_exceptions_flags(litehtml OFF)
qt_disable_warnings(litehtml)
qt_disable_warnings(gumbo)
 
When litehtml is found on the system, CMake refuses to set target properties on such targets:

 
[cmake] CMake Error at qtbase/cmake/QtFlagHandlingHelpers.cmake:189 (target_compile_definitions):
[cmake] Cannot specify compile definitions for target "litehtml" which is not built
[cmake] by this project.
[cmake] Call Stack (most recent call first):
[cmake] qttools/src/assistant/CMakeLists.txt:35 (qt_internal_set_exceptions_flags)
[cmake]
[cmake]
[cmake] CMake Error at qtbase/cmake/QtFlagHandlingHelpers.cmake:208 (get_target_property):
[cmake] get_target_property() called with non-existent target "litehtml".
[cmake] Call Stack (most recent call first):
[cmake] qttools/src/assistant/CMakeLists.txt:36 (qt_disable_warnings)
[cmake]
[cmake]
[cmake] CMake Error at qtbase/cmake/QtFlagHandlingHelpers.cmake:208 (get_target_property):
[cmake] get_target_property() called with non-existent target "gumbo".
[cmake] Call Stack (most recent call first):
[cmake] qttools/src/assistant/CMakeLists.txt:37 (qt_disable_warnings)
[cmake]
[cmake]
 
uninstalling litehtml from system resolves the error.