GenerateHelptags: Fix checking of DESTDIR

This should fix #1368
Since `include(InstallHelpers)` is included in CMakeLists.txt before `install(SCRIPT ${CMAKE_MODULE_PATH}/GenerateHelptags.cmake)` can we just use PREFIX here instead? E.g.:

```
   file(TO_CMAKE_PATH
     "${PREFIX}/share/nvim/runtime/doc"
     HELPTAGS_WORKING_DIRECTORY)
```

then we don't need the if/else block anymore. InstalleHelpers.cmake would still need to be modified as suggested by @xu-cheng .