test if a variable is empty and give default value during qmake

How can you test if a variable is empty or not defined in a qmake .pro file? I want to be able to set up a default value if the variable is not defined.

I tried

eval("VARIABLE" = ""){
     VARIABLE = test
}

eval("VARIABLE" = ""){
     message(variable is empty)
}

but I still get the message "variable is empty".