Fix handling of per-target flags in link rules.

Fix handling of per-target flags in link rules.
* automake.in (define_per_target_linker_variable): New function.
(handle_programs, handle_ltlibraries): Use it.
(%link_languages): New map.
(register_language): Fill it.
* lib/am/ltlibrary.am (%LTLIBRARY%): Do not append
$(%XLTLIBRARY%_LDFLAGS) to the command, this is now done by
define_per_target_linker_variable if needed.
* lib/am/program.am (%PROGRAM%%EXEEXT%): Likewise with
$(%XPROGRAM%_LDFLAGS).
* doc/automake.texi (Program and Library Variables): Mention
AM_LDFLAGS and AM_LIBTOOLFLAGS in the definition of maude_LDFLAGS
and maude_LIBTOOLFLAGS.
* tests/libtool9.test: New file.
* tests/Makefile.am (TESTS): Add it.
* NEWS: Explain the backward incompatibility.
Report from Akim Demaille.