configure: Put user flags after the mode flags
configure: Put user flags after the mode flags

It is sometimes convenient to build with flags that don't match any
existing mode.

Recently I was tracking a bug that would not reproduce with debug, but
reproduced with dev, so I tried debugging the result of

./configure.py --cflags="-O0 -g"

While the binary had debug info, it still had optimizations because
configure.py put the mode flags after the user flags (-O0 -O1). This
patch flips the order (-O1 -O0) so that the flags passed in the
command line win.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200116012318.361732-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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