Fix pch run

cmake/macros/FindPCHSupport.cmake

@@ -26,8 +26,15 @@ FUNCTION(GET_COMMON_PCH_PARAMS TARGET_NAME_LIST PCH_HEADER PCH_FE INCLUDE_PREFIX
     CollectIncludes(${TARGET_NAME} TARGET_INCLUDES TARGET_DEFINITIONS)
   ENDFOREACH()
 
-  LIST(REMOVE_DUPLICATES TARGET_INCLUDES)
-  LIST(REMOVE_DUPLICATES TARGET_DEFINITIONS)
+    IF (PARENT_PACKAGE_INCLUDE_DIRS)
+      LIST(REMOVE_DUPLICATES TARGET_INCLUDES)
+    ENDIF()
+    IF (PARENT_PACKAGE_LIBRARY_DIRS)
+      LIST(REMOVE_DUPLICATES TARGET_DEFINITIONS)
+    ENDIF()
+    # NOTE: Above, in the rare case that none of the subpackages contain any
+    # libraries or any include directories, we need to not call
+    # LIST(REMOVE_DUPLICATES ...).
 
   FOREACH(ITEM ${TARGET_INCLUDES})
     LIST(APPEND INCLUDE_FLAGS_LIST "${INCLUDE_PREFIX}\"${ITEM}\" ")