Determine if variable exists on make side from sh side

[PATTON, BILLY \(SBCSI\)]

Here is what I have:

all :
        @list=REFRESH+ldb+celltools+pub ;\
        if ! [ -z "$($list)" ] ; then \
                echo "list = '$$list'"; \
        fi


I need to determine if the variable REFRESH+ldb+celltools+pub exists
in the make world.  This name is normally  generated for values passed
in to a define.
I believe that the -z will tell if a sh variable has no value, but that
refers to a sh variable.
I need for sh functions to determine if a make variable exists.