[cmake-developers] CMake 3.13 - Generated moc files not added to VStudio solution

[Sebastian Holtermann]

> Any luck? I can try helping ya trace it if you need it.
> 

I traced this down to some degree.
The problem arises from the handling of file path for GENERATED vs non 
GENERATED files.
When the full path for a file is computed in cmSourceFile and the file doesn't
exist (mocs_compilation.cpp doesn't, yet), a path error flag is set.
The flag won't be raised when the file has the property GENERATED set, which
mocs_compilation.cpp hasn't at that point.

A quick solution in your CMakeLists.txt is to mark mocs_compilation.cpp
GENERATED first like this

```
set_property(SOURCE "${MOC_COMPILATION}" APPEND_STRING PROPERTY GENERATED "1")
set_property(SOURCE "${MOC_COMPILATION}" APPEND_STRING PROPERTY COMPILE_FLAGS 
" /FI\"${PrecompiledHeader}\"")
```

This works here for a small example project.

I'm too busy right now to address this in the CMake sources but I'll try to 
look at it next week.

-Sebastian