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 <[email protected]>

Author: Marti Bolivar

cmake/extensions.cmake

@@ -680,7 +680,7 @@ endmacro()
 # See 3.1 *_ifdef
 function(set_ifndef variable value)
   if(NOT ${variable})
-    set(${variable} ${value} PARENT_SCOPE)
+    set(${variable} ${value} ${ARGN} PARENT_SCOPE)
   endif()
 endfunction()