Newer binutils 'ar' has added an option 'D' for deterministic
builds, and at least on rawhide, this option is enabled by default.
But it conflicts with the 'u' optimization where the linker only
modifies libraries based on file timestamps, but can result in
different library ordering based on which files were touched last.
Thus, it results in some noisy compilation, for every CCLD line:

  CCLD     libvirt_driver_qemu_impl.la
  ar: `u' modifier ignored since `D' is the default (see `U')

Upstream automake has decided that defaulting ARFLAGS to 'cru' is
no longer beneficial, and that switching the default to 'cr' will
both silence the noise and not penalize modern build systems.

https://lists.gnu.org/archive/html/automake-patches/2015-06/msg00000.html

But rather than wait for newer automake to propagate to all systems
that already have newer binutils, we might as well just use the new
default ourselves, even on older platforms.

* configure.ac: Default AR[_]FLAGS to 'cr', not 'cru'.
Signed-off-by: NEric Blake <eblake@redhat.com>