[CMake] always execute CUSTOM_COMMAND

[msk.conf at seznam.cz]

> Here's a CMakeLists.txt that I used:
>
> PROJECT(test NONE)
> ADD_CUSTOM_TARGET(
>    "file.txt" ALL
>    COMMAND "touch" ARGS "file.txt"
>    VERBATIM
> )
> INSTALL(
>    FILES "${CMAKE_BINARY_DIR}/file.txt"
>    DESTINATION "/tmp"
> )
>
> -- Jack


Thanks a lot.

---
Dusan