6.9 Undefining Variables
========================

If you want to clear a variable, setting its value to empty is usually
sufficient. Expanding such a variable will yield the same result (empty
string) regardless of whether it was set or not. However, if you are
using the `flavor' (*note Flavor Function::) and `origin' (*note Origin
Function::) functions, there is a difference between a variable that
was never set and a variable with an empty value.  In such situations
you may want to use the `undefine' directive to make a variable appear
as if it was never set. For example:

     foo := foo
     bar = bar

     undefine foo
     undefine bar

     $(info $(origin foo))
     $(info $(flavor bar))

   This example will print "undefined" for both variables.

   If you want to undefine a command-line variable definition, you can
use the `override' directive together with `undefine', similar to how
this is done for variable definitions:

     override undefine CFLAGS