exch: fix compile error if renameat2 is not present

exch: fix compile error if renameat2 is not present

https://www.man7.org/linux/man-pages/man2/rename.2.html
Available since Linux 3.15 or glibc 2.28,
add a fallback for older systems.

My initial implementation did the exchange using a swap
file if both HAVE_RENAMEAT2 and SYS_renameat2 were absent.
But that is not really what is desired since exch is supposed
to be atomic. So simply return -1 renameat2 is available.

Signed-off-by: Thomas Devoogdt <thomas.devoogdt@barco.com>