Allow override of CFLAGS

If you attempt to specify your own CFLAGS on the command line (such as
make CFLAGS="--whatever"), this would prevent -DFIO_VERSION from being
added to CFLAGS. This patch uses the override directive to allow the
flag to be appended to CFLAGS.

Signed-off-by: Jens Axboe <axboe@kernel.dk>