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.

NEWS

@@ -8,6 +8,10 @@ NEWS - list of user-visible changes between releases of GNU Libtool
     variable, which obsoletes AR_FLAGS.  This is due to naming conventions
     among other *FLAGS and to be consistent with Automake's ARFLAGS.
 
+** Important incompatible changes:
+
+  - Libtool changed ARFLAGS/AR_FLAGS default from 'cru' to 'cr'.
+
 ** Bug fixes:
 
   - Fix significant slowdown of libtoolize for certain projects (regression

doc/libtool.texi

@@ -602,7 +602,7 @@ Without libtool, the programmer would invoke the @command{ar} command to
 create a static library:
 
 @example
-burger$ @kbd{ar cru libhello.a hello.o foo.o}
+burger$ @kbd{ar cr libhello.a hello.o foo.o}
 burger$
 @end example
 
@@ -632,7 +632,7 @@ libtool are the same ones you would use to produce an executable named
 a23$ @kbd{libtool --mode=link gcc -g -O -o libhello.la foo.o hello.o}
 *** Warning: Linking the shared library libhello.la against the
 *** non-libtool objects foo.o hello.o is not portable!
-ar cru .libs/libhello.a
+ar cr .libs/libhello.a
 ranlib .libs/libhello.a
 creating libhello.la
 (cd .libs && rm -f libhello.la && ln -s ../libhello.la libhello.la)
@@ -662,7 +662,7 @@ archive, not a shared library (@pxref{Static libraries}).}:
 @example
 a23$ @kbd{libtool --mode=link gcc -g -O -o libhello.la foo.lo hello.lo \
                 -rpath /usr/local/lib -lm}
-ar cru @value{objdir}/libhello.a foo.o hello.o
+ar cr @value{objdir}/libhello.a foo.o hello.o
 ranlib @value{objdir}/libhello.a
 creating libhello.la
 (cd @value{objdir} && rm -f libhello.la && ln -s ../libhello.la libhello.la)
@@ -676,7 +676,7 @@ burger$ @kbd{libtool --mode=link gcc -g -O -o libhello.la foo.lo hello.lo \
                 -rpath /usr/local/lib -lm}
 rm -fr  @value{objdir}/libhello.a @value{objdir}/libhello.la
 ld -Bshareable -o @value{objdir}/libhello.so.0.0 @value{objdir}/foo.o @value{objdir}/hello.o -lm
-ar cru @value{objdir}/libhello.a foo.o hello.o
+ar cr @value{objdir}/libhello.a foo.o hello.o
 ranlib @value{objdir}/libhello.a
 creating libhello.la
 (cd @value{objdir} && rm -f libhello.la && ln -s ../libhello.la libhello.la)
@@ -6001,7 +6001,7 @@ in cases where it is necessary.
 @subsection Archivers
 
 On all known systems, building a static library can be accomplished by
-running @kbd{ar cru lib@var{name}.a @var{obj1}.o @var{obj2}.o @dots{}},
+running @kbd{ar cr lib@var{name}.a @var{obj1}.o @var{obj2}.o @dots{}},
 where the @file{.a} file is the output library, and each @file{.o} file is an
 object file.
 

m4/libtool.m4

@@ -1042,8 +1042,8 @@ int forced_loaded() { return 2;}
 _LT_EOF
       echo "$LTCC $LTCFLAGS -c -o conftest.o conftest.c" >&AS_MESSAGE_LOG_FD
       $LTCC $LTCFLAGS -c -o conftest.o conftest.c 2>&AS_MESSAGE_LOG_FD
-      echo "$AR cru libconftest.a conftest.o" >&AS_MESSAGE_LOG_FD
-      $AR cru libconftest.a conftest.o 2>&AS_MESSAGE_LOG_FD
+      echo "$AR $AR_FLAGS libconftest.a conftest.o" >&AS_MESSAGE_LOG_FD
+      $AR $AR_FLAGS libconftest.a conftest.o 2>&AS_MESSAGE_LOG_FD
       echo "$RANLIB libconftest.a" >&AS_MESSAGE_LOG_FD
       $RANLIB libconftest.a 2>&AS_MESSAGE_LOG_FD
       cat > conftest.c << _LT_EOF
@@ -1501,7 +1501,7 @@ _LT_DECL([], [AR], [1], [The archiver])
 # ARFLAGS for automake and AR_FLAGS for libtool).  FIXME: Make the AR_FLAGS
 # variable obsoleted/removed.
 
-test ${AR_FLAGS+y} || AR_FLAGS=${ARFLAGS-cru}
+test ${AR_FLAGS+y} || AR_FLAGS=${ARFLAGS-cr}
 lt_ar_flags=$AR_FLAGS
 _LT_DECL([], [lt_ar_flags], [0], [Flags to create an archive (by configure)])