[CMake] Improve ROOT_GENERATE_DICTIONARY

- In case ROOT_GENERATE_DICTIONARY is invoked with a dependency that
doesn't have a dictionary itself, the for loop through dependencies now
just continue()s. Before, this would raise a CMake error.
- The object library with the dictionary file is now linked into the
main library using target_link_libraries().
- When the list of include directories for the dictionary is generated,
the INTERFACE_SYSTEM_INCLUDE_DIRECTORIES of the dependencies is now
honoured. Before, system includes would decay to normal includes.

Unfortunately, PRIVATE includes still decay to normal -I includes. This
can lead header conflicts when ROOT is built while another ROOT is installed
in system include directories, but only for the dictionary files.
Since ROOT include directories are very generously prepended to all targets,
I wasn't able to provoke a header conflict.

Author: hageboeck

cmake/modules/RootMacros.cmake

@@ -562,6 +562,11 @@ function(ROOT_GENERATE_DICTIONARY dictionary)
   #---Get the library and module dependencies-----------------
   if(ARG_DEPENDENCIES)
     foreach(dep ${ARG_DEPENDENCIES})
+      if(NOT TARGET G__${dep})
+        # This is a library that doesn't come with dictionary/pcm
+        continue()
+      endif()
+
       set(dependent_pcm ${libprefix}${dep}_rdict.pcm)
       if (runtime_cxxmodules)
         set(dependent_pcm ${dep}.pcm)
@@ -648,7 +653,7 @@ function(ROOT_GENERATE_DICTIONARY dictionary)
   if(TARGET "${ARG_MODULE}" AND NOT "${ARG_MODULE}" STREQUAL "${dictionary}")
     add_library(${dictionary} OBJECT ${dictionary}.cxx)
     set_target_properties(${dictionary} PROPERTIES POSITION_INDEPENDENT_CODE TRUE)
-    target_sources(${ARG_MODULE} PRIVATE $<TARGET_OBJECTS:${dictionary}>)
+    target_link_libraries(${ARG_MODULE} PRIVATE ${dictionary})
 
     target_compile_options(${dictionary} PRIVATE
       $<TARGET_PROPERTY:${ARG_MODULE},COMPILE_OPTIONS>)
@@ -659,8 +664,14 @@ function(ROOT_GENERATE_DICTIONARY dictionary)
     target_compile_features(${dictionary} PRIVATE
       $<TARGET_PROPERTY:${ARG_MODULE},COMPILE_FEATURES>)
 
-    target_include_directories(${dictionary} PRIVATE
-      ${incdirs} $<TARGET_PROPERTY:${ARG_MODULE},INCLUDE_DIRECTORIES>)
+    target_include_directories(${dictionary} PRIVATE ${incdirs} $<TARGET_PROPERTY:${ARG_MODULE},INCLUDE_DIRECTORIES>)
+
+    # Above we are copying all include directories of the module, irrespective of whether they are system includes.
+    # CMake copies them as -I even when they should be -isystem.
+    # We can fix this for INTERFACE includes by also copying the respective property.
+    # For PRIVATE includes this doesn't work, but since all ROOT includes are prepended, it didn't do harm yet.
+    set_property(TARGET ${dictionary} APPEND PROPERTY
+      INTERFACE_SYSTEM_INCLUDE_DIRECTORIES $<TARGET_PROPERTY:${ARG_MODULE},INTERFACE_SYSTEM_INCLUDE_DIRECTORIES>)
   else()
     get_filename_component(dictionary_name ${dictionary} NAME)
     add_custom_target(${dictionary_name} DEPENDS ${dictionary}.cxx ${pcm_name} ${rootmap_name} ${cpp_module_file})