GNU make also supports a special variable. Note that any value
you assign to this variable will be ignored; it will always return its
special value.
The first special variable is .VARIABLES. When expanded, the
value consists of a list of the names of all global variables
defined in all makefiles read up until that point. This includes
variables which have empty values, as well as built-in variables
(see Variables Used by Implicit Rules), but
does not include any variables which are only defined in a
target-specific context.