Use AC_ARG_VAR to set ARFLAGS.

Summary:

The user can set ARFLAGS in the ./configure step with
  ./configure ARFLAGS=...
If he chooses not to do so, ARFLAGS will be set to cr.

Backport of core PR10766
https://github.com/bitcoin/bitcoin/pull/10766/files

Test Plan:

make

Check the warning:

/usr/bin/ar: `u' modifier ignored since `D' is the default (see `U')

is no longer displayed after this patch.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3544