cmake: extensions: support list arguments to set_ifndef()

I keep tripping over not being able to use set_ifndef to set a value
to a list, like this:

set_ifndef(VARIABLE value1 value2 value3...)

Allow that to work without changing its existing behavior when called
with one argument.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>
1 file changed