Merge #10766: Building Environment: Set ARFLAGS to cr

Merge #10766: Building Environment: Set ARFLAGS to cr

912da1dcc Use AC_ARG_VAR to set ARFLAGS. (René Nyffenegger)

Pull request description:

  Override the default of ARFLAGS of `cru` to `cr`.

  When building, ar produces a warning for each archive, for example
  ```
    AR       libbitcoin_server.a
  /usr/bin/ar: `u' modifier ignored since `D' is the default (see `U')

  ```
  Since `u` is the default anyway, it cannot hurt to remove it.

Tree-SHA512: 7466764f847b70f0f67db25dac87a7794477abf1997cb946682f394fe80ae86ac3ed52cbadb35f0c18a87467755bde5a5158430444cd26fb60fa363cc7bd486d