Drop support for DJGPP, MS-DOS, Windows 95/98/ME

Drop support for DJGPP, MS-DOS, Windows 95/98/ME

Basically a backport and squash-in of three Automake-NG commits:
v1.12.1-447-g041fe15, v1.12-343-g4526363, v1.12-341-gc853c20,
plus proper simplifications of the ylwrap script.

See also:
<http://lists.gnu.org/archive/html/automake-ng/2012-05/msg00104.html>

* automake.in (BEGIN): Don't override $ENV{SHELL} for DJGPP.
(handle_libtool): Do not bother cleaning '_libs' directories, they
are only used by Libtool on MS-DOS (and we no longer support that
setup).
* syntax-checks.mk (automake_diff_no): Adjust, as now there are only
seven (not eight) different lines between 'automake.in' and 'automake'.
* bootstrap.sh: Don't special case the default value for BOOTSTRAP_SHELL
on DJGPP.  While we are at, allow that variable to be overridden from
the environment. Also, adjust comments: we don't use "ln -s" not because
it's not properly supported by DJGPP, but because it might not be
properly supported by MinGW/MSYS.
* configure.ac (MODIFICATION_DELAY): Define to '2' unconditionally, as
we no longer have to cater to quirks of Windows 95/98/ME.
* lib/am/texibuild.am: Remove support for '*.iNN' files used on DJGPP.
* lib/am/texinfos.am: Likewise.
* lib/ylwrap: Remove support for DOS-specific filenames for bison
output.
* t/txinfo19.sh: Remove as obsolete.
* t/list-of-tests.mk: Adjust.
* NEWS: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>