configure: override AR_FLAGS to silence warning

The automake default ar flags are 'cru', but the 'u' flag in there
causes warnings on many modern Linux distros. Removing 'u' may have a
minor performance impact on older distros but should not cause harm.

Explained on the automake mailing list already back in April 2015:

https://www.mail-archive.com/automake-patches@gnu.org/msg07705.html

Reported-by: elephoenix on github
Fixes #2617
Closes #2619