ARFLAGS: use 'cr' instead of 'cru' by default

ARFLAGS: use 'cr' instead of 'cru' by default

In some GNU/Linux distributions people started to compile 'ar'
binary with --enable-deterministic-archives (binutils project).
That, however, in combination with our previous long time working
default AR_FLAGS=cru causes warnings on such installations:
ar: `u' modifier ignored since `D' is the default (see `U')

The 'u' option (at least with GNU binutils) did small optimization
during repeated builds because it instructed 'ar' to not
open/close unchanged *.o files and to rather read their contents
from old archive file.  However, its removal should not cause a
big performance hit for usual workflows.

Distributions started using --enable-deterministic-archives
knowing that it would disable the 'u', just to rather have a bit
more deterministic builds.

Also, to justify this change a bit more, keeping 'u' in ARFLAGS
could only result in many per-project changes to override
Libtool's ARFLAGS default, just to silent such warnings.

Fixes bug#19967.  Reported by Eric Blake.

* m4/libtool.m4 (_LT_PROG_AR): Default AR_FLAGS to 'cr'.
(_LT_REQUIRED_DARWIN_CHECKS): Use $AR_FLAGS instead 'cru' string.
* doc/libtool.texi: Do 's/ar cru/ar cr/' in whole documentation.
* NEWS: Document.