ade: configure: Don't modify caller-supplied cflags, ldflags in-place

Python arguments are passed by reference, so if we append to these
arrays, the caller's copy of the list is modified. This seems unlikely
to be intended.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Justin Kim <justin.kim@collabora.com>
Differential Revision: https://phabricator.apertis.org/D6773