[CMake] EXEC_PROGRAM question

[Doug Henry]

I am doing the following command in my top level CMakeLists.txt file:

EXEC_PROGRAM(
    date
    ARGS +%y%j
    OUTPUT_VARIABLE DATEVAR)

ADD_DEFINITIONS(-DDATESTR=${DATEVAR})


The thing I don't understand is that DATEVAR is only defined in the
top-level makefile.  It does not appear in other subdirs/projects. 
How do I get the DATEVAR variable to be cached and useable in a more
global way (e.g. like traditional SET statements)?