Redirect output of $AR check to /dev/null

Redirect output of $AR check to /dev/null

./configure logs this on Mac OS:

checking whether ar supports D option... /Library/Developer/CommandLineTools/usr/bin/ar: illegal option -- D
usage:  ar -d [-TLsv] archive file ...
	ar -m [-TLsv] archive file ...
	ar -m [-abiTLsv] position archive file ...
	ar -p [-TLsv] archive [file ...]
	ar -q [-cTLsv] archive file ...
	ar -r [-cuTLsv] archive file ...
	ar -r [-abciuTLsv] position archive file ...
	ar -t [-TLsv] archive [file ...]
	ar -x [-ouTLsv] archive [file ...]
no

This is quite noisy and we're only interested in the result of the
check, not the output.

Signed-off-by: Ruben Kerkhof <ruben@rubenkerkhof.com>