configure: Make -O flag configurable
configure: Make -O flag configurable

It was noticed, that current optimization levels do not
generate fast enough code for dev builds. On the other
hand just increasing the default optimization level will
make re-compile-mostly work much more frustrating.

The new configure.py option allows to select the desired
-O option value by hands. Current hard-coded values are
used as defaults.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla