CMake: fix build and install location of Qt help files

src/Doc/CMakeLists.txt

@@ -11,7 +11,7 @@ configure_file(CONTRIBUTORS
 if(BUILD_GUI)
     FILE(GLOB QCH_FILES freecad.qch.part*)
     list(SORT QCH_FILES)
-    set (DEST_FILE ${CMAKE_BINARY_DIR}/doc/freecad.qch)
+    set (DEST_FILE ${CMAKE_BINARY_DIR}/${CMAKE_INSTALL_DOCDIR}/freecad.qch)
     foreach(SRC_FILE ${QCH_FILES})
         if(${SRC_FILE} IS_NEWER_THAN ${DEST_FILE})
             EXECUTE_PROCESS(COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_SOURCE_DIR}/src/Tools/catfiles.py
@@ -22,9 +22,9 @@ if(BUILD_GUI)
         endif()
     endforeach()
 
-    configure_file(freecad.qhc ${CMAKE_BINARY_DIR}/doc/freecad.qhc COPYONLY)
+    configure_file(freecad.qhc ${CMAKE_BINARY_DIR}/${CMAKE_INSTALL_DOCDIR}/freecad.qhc COPYONLY)
     INSTALL(FILES
-        ${CMAKE_BINARY_DIR}/doc/freecad.qch
+        ${DEST_FILE}
         DESTINATION ${CMAKE_INSTALL_DOCDIR}
     )
 endif(BUILD_GUI)