[CMake] how to specify dependencies out of the source tree?

[Rolf Eike Beer]

> I thank you so much Eric,
> However I managed this thing and obteined the expected result by writing
> something similar and then including it in my root CMakeLists.txt:
>
> ----- util lib dependency -----
> # -*- Makefile -*-
>
> IF(BUILD_STANDALONE)

For those project I need that I usually do this without the user need to
do something special by:

IF (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)

Eike