Fix 32-bit compile warning in should_check_rate()

Introduced with the change from unsigned long -> uint64_T

Signed-off-by: Jens Axboe <axboe@kernel.dk>
configure: add --cc= option to script

Get rid of uname -o usage for Windows, and specifically require
CC=bla ./configure or ./configure --cc=bla to set something else
than gcc.

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