configure: use pkg-config to find liburing

configure: use pkg-config to find liburing

Change our autoconf macros so that instead of checking for the presence
of liburing.h, we use pkg-config.

The benefit of this is that we can then check the version of liburing,
and do conditional compilation based on this. There's a macro
IO_URING_CHECK_VERSION already, but it's only in relatively recent
versions of liburing.h.

This replaces HAVE_URING_H, defined by AC_CHECK_HEADERS, with
HAVE_URING. I also had to rename PKG_{MAJOR,MINOR,REVISION,BUILD} to
start with PACKAGE_, to avoid "possibly undefined macro" errors; it
looks like pkg-config assumes that anything called PKG_* is for its own
use.

Signed-off-by: Mark Harmstone <maharmstone@fb.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>