lupdate behaves weird on different OS

Hello folks,

I found this weird behavior of lupdate tool during creating my strict CI automation: the sources have to be completely prepared for build, so I do not expect any updates and make the src directory read only. After that I run `qt-cmake src -B build` and `cmake --build build` commands (in general) and expect it to be done successfully if everything is fine and fail if some translation is incorrect or not updated.

The issue is:

• On windows lupdate tries to write ts files even if they are no need in modification of ts files. If they are not requiring modification - lupdate should not try to write them.
• On the other systems (macos, linux) it looks like lupdate doesn't try to update the ts files at all - even if there is missed messages (and this is a bug too, because developer have to update them if the sources was updated with new messages).

I tested it on 3 sample applications (console, widgets, quick) that is available if you create the new project in QtCreator (6.0.2). They contain just empty ts files like that by default:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE TS>
<TS version="2.1" language="ru_RU"></TS>

• During build widgets on windows lupdate tries to make it like that:

  <?xml version="1.0" encoding="utf-8"?>
  <!DOCTYPE TS>
  <TS version="2.1" language="ru_RU">
  <context>
      <name>MainWindow</name>
      <message>
          <location filename="mainwindow.ui" line="14"/>
          <source>MainWindow</source>
          <translation type="unfinished"></translation>
      </message>
  </context>
  </TS>
  

• For console app build on windows - no changes on windows, but still tries to write the file.

• For quick app build on windows it surprisingly do not try to modify anything. I checked the differences in CMakeFile.txt and seems it's not setting the translation files so they just skipped (another bug, but in qtcreator I suppose)

So folks, could you please check and make it more clear for me?

Thank you