BUG #19368: f54af9f does not create the correct macro for autotools build

The following bug has been logged on the website:

Bug reference:      19368
Logged by:          Evan Si
Email address:      evsi@amazon.com
PostgreSQL version: 18.1
Operating system:   Amazon Linux 2, Amazon Linux 2023
Description:

https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=f54af9f2679d

The commit f54af9f2679d added a macro HAVE_LIBURING_QUEUE_INIT_MEM that is
checked to see if slowdown from memory mappings can be avoided when using
io_uring.
This HAVE_LIBURING_QUEUE_INIT_MEM is set by Meson build after checking
liburing.
However, the autotools build seems to set a different macro,
HAVE_IO_URING_QUEUE_INIT_MEM, which is never used.

This means when the server is built with autotools, the slowdown when using
io_uring can never be avoided.