liburing: specifying --prefix to configure script was ineffective, fix.
liburing: specifying --prefix to configure script was ineffective, fix.

The configure script used the default value of the prefix variable
(/usr) to determine the includedir etc. *before* parsing the command
line to determine if the user specified a prefix. This made configure
--prefix ineffective.

Fix.

Tested with configure --help, configure --prefix, configure --prefix
--includedir

Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing