Fix developer documentation build

Amends 6ec3c8f9c9

We may not remove the generator expressions, they are actually expanded
and used, and important, e.g.

    $<TARGET_PROPERTY:Qt::Core,INTERFACE_INCLUDE_DIRECTORIES>

The original issue was that the target that was referred to for some of
these (Qt6::QmlDomPrivate) was not known at the point where the
documentation target needed them.

Just filter out non-existent targets instead.

Change-Id: If75b2f06fdcefdfad1b2ab0ece3d994e3000edfd
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

doc/CMakeLists.txt

@@ -16,8 +16,9 @@ function(_find_all_includes _ret_includes _ret_framework_paths)
       string(FIND "${_include}" "/src/plugins/" _in_plugins)
       string(FIND "${_include}" "/src/libs/" _in_libs)
       string(FIND "${_include}" "${CMAKE_BINARY_DIR}" _in_build)
-      if(_in_plugins LESS 0 AND _in_libs LESS 0 AND _in_build LESS 0)
-        remove_generator_expressions(_include ${_include})
+      string(REGEX MATCH "\\$<TARGET_PROPERTY:([^,]+).*>" _property_match "${_include}")
+      set(_property_target "${CMAKE_MATCH_1}")
+      if(_in_plugins LESS 0 AND _in_libs LESS 0 AND _in_build LESS 0 AND (NOT _property_target OR TARGET ${_property_target}))
         list(APPEND _all_includes ${_include})
       endif()
     endforeach()