configure: fixup build so that it's more resilient

Allow people to clear/set EXTFLAGS/OPTFLAGS at will.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
axmap: fix bug with max_bs/min_bs ratio being > 64

Signed-off-by: Jens Axboe <axboe@kernel.dk>
t/axmap: add test for multi bit sets

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