[CMake] file dependency not working in external projects

[hex]

hello,

I have a problem with a build step.

The following command is run every time somefile changes:

add_custom_command( OUTPUT out.put
     COMMAND touch out.put
     DEPENDS somefile.txt
)

add_custom_target( sometarget DEPENDS out.put )


I move this snippet inside an external project (with 
ExternalProject_Add) and the custom command is only run once at build.

I am using absolute path to somefile, the command is not rebuild 
changing somefile.

I am using Gnu make generator and build with make all


What am I missing?