ar-lib: new 'AM_PROG_AR' macro, triggering the 'ar-lib' script

ar-lib: new 'AM_PROG_AR' macro, triggering the 'ar-lib' script

* m4/ar-lib.m4: New macro AM_PROG_AR, which locates an
archiver and triggers the auxiliary 'ar-lib' script if needed.
* m4/Makefile.am (dist_m4data_DATA): Update.
* automake.in ($seen_ar): New variable.
(scan_autoconf_traces): Set it.
(handle_libraries, handle_ltlibraries): Require AM_PROG_AR for
portability.
* doc/automake.texi (Public Macros): Mention the new
'AM_PROG_AR' macro.
(Subpackages): Add AM_PROG_AR to the example.
(A Library): Adjust recommendations for AR given the new
AM_PROG_AR macro.
* All relevant tests: Adjust to new portability requirements due
to the new AM_PROG_AR macro.
* tests/ar-lib2.test: New test, checking that AM_PROG_AR triggers
install of ar-lib.
* tests/ar-lib3.test: New test, checking that lib_LIBRARIES
requires AM_PROG_AR.
* tests/ar-lib4.test: New test, checking that lib_LTLIBRARIES
requires AM_PROG_AR.
* tests/ar-lib5a.test: New test, checking that AM_PROG_AR triggers
use of ar-lib when the archiver is Microsoft lib.
* tests/ar-lib5b.test: New test, checking that AM_PROG_AR triggers
use of ar-lib when the archiver is a faked lib.
* tests/ar-lib6a.test: New test, checking the ordering of
AM_PROG_AR and LT_INIT.
* tests/ar-lib6b.test: New test, checking the ordering of
AM_PROG_AR and AC_PROG_LIBTOOL.
* tests/ar-lib7.test: New test, checking that automake warns
if ar-lib is missing.
* tests/ar3.test: New test, checking that AR and ARFLAGS may
be overridden by the user even if AM_PROG_AR is used.
* tests/ar4.test: New test, checking that AM_PROG_AR bails out
if it cannot determine the archiver interface.
* tests/ar5.test: New test, checking that AM_PROG_AR runs its
optional argument if it cannot determine the archiver interface.
* tests/defs.in: New required entry 'lib'.
* tests/Makefile.am (TESTS): Update.
* NEWS: Update.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>