configure.py: rename warning_supported to flag_supported
configure.py: rename warning_supported to flag_supported

warning_supported() can be used to detect support of any compiler flag,
not just warnings.

duarten/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla